Unmanaged switch generally does not support VLAN. A managed switch can do that.

the CSS326 switch has 24 ports and POE, you can use them for PPPoE. Then the only thing you need to do is connecting the switch to the router through its SFP+ port. This 1000BASE-T SFP copper transceiver can help you do that https://www.fs.com/products/75322.html

Why connect two ISP to one router? I do not think this will work. If you use the SFP+ for ISP, the Combo port and Ethernet ports can only be used to connect Ethernet devices.

Why not place the 2 antenas in different places?

This will work? I'll try.

How many devices will you connect to the router? Routers often have less than eight ports (often four ports). If you need eight or more ports, a poe switch is beter choice.

Sure you can use it to connect to a Cisco switch. but you need to buy SFP transceivers work both in Mikrotik and Cisco devices. i think this will meet your need https://www.fs.com/products/37199.html

yes it will work, but ia suggest SFP cable for gagibit devices.

I do not think the RJ45 sfp+ module can be used in your RB2011. It only has one SFP port. So you may connect your RB2011 to gigabit switch through SFP modules.

yes, 10G SFP+ cable is back compatible to 1g sfp. but it's best choice to use a sfp cable.

They each has their own advantages. Mikrotik is simpler and cheapter for me.

Why not use both a router and a poe switch (a 8 port switch will be enough). This enables you to have enough ports for future needs.