I have a client that represents an RV park. They're looking to build several cabins (between 8 and 10 of them) that will be entirely made out of metal.

They have existing Ruckus WiFi APs throughout the park from a separate vendor.

They have built one of the cabins already, and as expected the WiFi is basically unusable inside it. Signal right outside the building is -50ish dBm on 5Ghz and -40ish on 2.4Ghz as measured from a laptop. From where they plan to build the remaining cabins, I'd expect signal outside the building to be about the same on each of them.

What's a good Mikrotik-based solution to accommodate them? Would something like a SXTsq Lite5 outside to handle the point-to-point to the existing APs connected to something like a hAP/mAP/whatever, be a viable solution? Should I consider a device that will allow 2 separate WiFi cards (one of inside, and one with antennas that would be outside)?

Useable bandwidth requirements are quite low. Each cabin can only accommodate 2 people max, and each client is currently throttled to 20Mbps.

I understand that a 60Ghz solution for the PtP would be ideal, but given that there is sufficient 5/2.4Ghz signal outside the cabins, and the highest possible bandwidth and lowest latency aren't really required, I'd rather do this using the existing WiFi APs.

Ideally you'd use dual-band device with separate detachable antennae for both bands (at least one band has to utilize detachable antennae so you can place them outside, indoor band can use built-in antennae). Then use one band outside as backhaul and another band inside as AP for guests. From capacity point of view it'd be better to use 5GHz for backhaul and 2.4GHz for in-cabin coverage.
I'm not sure if MT sells a suitable device though.

Using device with two radios on same band (again separate detachable antennae) would work in this case as well but you'd still have to be careful to place inside snd outside antennae so that there woikd slways be metal wall/ceiling in the way to improve separation. Antenna cables would have to be router as far apart as possible to minimize cross-talk.
Variant with different bands is definitely easier to do.
I guess this kind of device is strictly BYOD in MT's portfolio.

Of course it's possible to use a pair of devices per cabin, one outdoor as back-haul and one indoor, connected with UTP cable (used for PoE for outdoor device as well). Again I suggest to use different bands for both purposes if feasible. This setup would likely be more expensive, but it would be more "off-the-shelf" type of project which would likely prove better in mid-term due to easier maintenance. And you have a wider choice of devices to use.

is that like a mobil home idea but made out of metal containers? if yes - just make a central cable connection.
where i worked, each mobil home had a lan cable going in together with the power cable, and we had a distribution point every 4 mobil homes (so about 60-75m in distance). each mobil home had a ruckus ap behind the tv, handling iptv, domotica and wifi.
can't you make a similar solution? if there's no cabling for internet, just mount a cpe on a pole where the power distribution point is and a poe switch to handle the in-mobile home APs. easier, better, cheaper.

Using device with two radios on same band (again separate detachable antennae) would work in this case as well but you'd still have to be careful to place inside snd outside antennae so that there woikd slways be metal wall/ceiling in the way to improve separation. Antenna cables would have to be router as far apart as possible to minimize cross-talk.
Variant with different bands is definitely easier to do.
I guess this kind of device is strictly BYOD in MT's portfolio.

I'm thinking that with an AP with a directional antenna (such as the SXTsq) and the fact that the building is completely metal would limit interference. Of course, there's interferance no matter what you do, since it's an RV park full of people using who-knows-what devices

Of course it's possible to use a pair of devices per cabin, one outdoor as back-haul and one indoor, connected with UTP cable (used for PoE for outdoor device as well). Again I suggest to use different bands for both purposes if feasible. This setup would likely be more expensive, but it would be more "off-the-shelf" type of project which would likely prove better in mid-term due to easier maintenance. And you have a wider choice of devices to use.

Other than the radio interferance, is there any issue using 2 WiFi cards on a single device? For example the NetMetal AC2 has a built-in WiFi card an an additional PCIe slot.

is that like a mobil home idea but made out of metal containers? if yes - just make a central cable connection.
where i worked, each mobil home had a lan cable going in together with the power cable, and we had a distribution point every 4 mobil homes (so about 60-75m in distance). each mobil home had a ruckus ap behind the tv, handling iptv, domotica and wifi.
can't you make a similar solution? if there's no cabling for internet, just mount a cpe on a pole where the power distribution point is and a poe switch to handle the in-mobile home APs. easier, better, cheaper.

I don't know that that would really be "cheaper." Maybe over 15 years, but my relationship with them won't last that long in all liklihood. Around here, the cost to have a pole stood up is about $3,000, and that's assuming we can find contractors that even want to do the job.

It's all hard-top surface, and directional boring is stupid money here as well.

Other than the radio interferance, is there any issue using 2 WiFi cards on a single device?

As long as additional card is supported by ROS there should not be any problems other than possibly destructive interference from the other radio.

Outdoor CPE and indoor AP, there is little chance for interference. Two Wi-Fi cards on one device will interfere/cross-talk with one another unless you can find some crazy shielded cards.

is that like a mobil home idea but made out of metal containers? if yes - just make a central cable connection.
where i worked, each mobil home had a lan cable going in together with the power cable, and we had a distribution point every 4 mobil homes (so about 60-75m in distance). each mobil home had a ruckus ap behind the tv, handling iptv, domotica and wifi.
can't you make a similar solution? if there's no cabling for internet, just mount a cpe on a pole where the power distribution point is and a poe switch to handle the in-mobile home APs. easier, better, cheaper.

I don't know that that would really be "cheaper." Maybe over 15 years, but my relationship with them won't last that long in all liklihood. Around here, the cost to have a pole stood up is about $3,000, and that's assuming we can find contractors that even want to do the job.

It's all hard-top surface, and directional boring is stupid money here as well.

not sure we understood each other well... my "pole" being the usual 3m high metal pole that would reach over the roof of the mobil homes to a central AP. here they cost like 50eur each, plus a bit of concrete to hold them in place. not sure what your idea of "pole" was.
point being - you don't want a routed network, you want a bridged network connected to the existing ruckus. your idea of using mikrotik for outdoor-indoor wifi is a shot in the leg because you can't bridge that. mikrotik supports bridging only with mikrotik wifi - and your client uses ruckus. you simply can't "wirelessly connect" a mikrotik to ruckus, and expect to have the same subnet on the other side. it's not possible.
so each mobil home would have a mikrotik that would work in station mode, have it's own dhcp server, and it's own wifi network. multiply that by the number of such mobil homes. it's a big no-no. you'll create a mess out of a working network.
your only options are either - create a separate mikrotik based bridge network only for indoor wifi - or drop the idea of using mikrotik for this and get ruckus, because that way the FT will work normally.
personally, while i worked for my ex company we had close to a thousand aps in the camp, and we ditched all mismatched vendors, including mikrotik, cisco, and airlive (!?), for a duo of vendors that offered a working FT solution at the time. and it did work. our network was bridged completely, with a central dhcp server.
you're trying to offer a solution to a problem but with a completely wrong idea of that solution. ditch the "repeater" thingy idea, get cabling inside each of those metal cages and use a vendor that has a compatible FT to ruckus. or ruckus. if you do anything other than that - your client will eat you alive. i know i would.
or drop the idea of offering that client your solution if you only offer mikrotik.