My subnet is rather large 10.0.0.0/16 (no, I don't need 65k clients, but it's convenient to use 10.0.A.0, 10.0.B.0, ... for various categories / persons). I also have a working WireGuard Road Warrior configuration in a dedicated 172.17.20.0.1/24 subnet.
I wouldn't mind to keep it as it is right but there is one issue. The WG peers cannot connect to SMB on Windows machines as the firewall allows only the local subnet (in my understanding 10.0.0.0/16), turning off the Windows firewall doesn't seem like a good idea (as some of the device are mobile device) although it works and doesn't really scale as I'd have to either turn off or change the firewalls of all the Windows devices...
So I'd like the WG peers to be part of the 10.0.0.0/16 network (say 10.0.X.0). I'm just unsure how to achieve that.
I read in viewtopic.php?p=978961#p978961 that in theory it could work, but I really don't know how this should be done. (My home-grade router of a different brand had an IPsec setup which "just" worked. I know that WireGuard is a different beast, but maybe there is a way...)

Any help would be greatly appreciated (last time I did a bigger change I borked it and had to use Woobm-USB to get back online )

As stated in the topic it should be possible in theory if you allocate addresses from the subnet, i.e. create a new subnet which is part of the original one. For instance:
Only thing which I think was not said is to enable proxy-arp for the bridge provided the 10.0.0.0/16 address is set on such one

I don't think what you want to achieve (WG Road Warriors being able to access the SMB shares) will work if you let the WG peers be part of the 10.0.0.0/16 subnet. It worked for the post you linked to, because they used the workaround to access the 10.2.0.0/16 subnet from the 10.1.0.0/16 subnet.

Let's assume that you have successfully set the WG peers to be in a 10.0.50.0/24 subnet. When a WG client from this subnet access something on the LAN subnet 10.0.0.0/16 (an SMB server for instance), the router will happily route the packet sent by the WG peer to the destination. And the Windows Server will accept the packet (because it sees the source IP address to be in the allowed 10.0.0.0/16 subnet). But what do you think will happen when the Windows Server replies? It will be replying to an address in the 10.0.0.0/16 subnet, as a result it will completely bypass the router and only use ARP to try to find the MAC address of the IP address and only try to do everything on the same Layer 2 network. It won't be able to send the packet back to the WG peer, because that requires the Windows Server to use the router as gateway to forward the packet. Which is not attempted if the destination is in the same configured subnet 10.0.0.0/16.

What should work is that you keep your WG peers in the 172.17.20.0/24 subnet but then add a srcnat rule to the firewall, to masquerade connection from 172.17.20.0/24 into 10.0.0.0/16 (similar to what is done for Hairpin NAT). That way, when client from 172.17.20.0/24 access 10.0.0.0/16, the source address of the packet sent will be changed to the 10.0.0.1 address of the router, which means the SMB server will accept the connection. When they answer, packets will be sent back to the router at 10.0.0.1, which will then undo the NAT and forward them to the original host within 172.17.20.0/24.

Makes sense to me CGNX. Good advice!!
The other possibility is to use zerotier I suppose.