The following can be done and is usually near enough.
The router hosting wireguard does not need to be the gateway.
eg. Lan Using 192.168.0.0/24
Though if you are using a very common IP address range like above, you might want to consider renumbering your Lan.
Note: The router needs an IP address in the range on its Lan port. (eg. 192.168.0.10/24)
Grab a subnet range out of the Lan that is not in use. To use for your wireguard. Make sure DHCP won’t assign IP addresses in this range.
eg. 192.168.0.224/28 (.224 to .239, 16 addresses, 15 clients)
Create your wireguard interface, give it the ip address 192.168.0.224/28 <= Sets up required routing.
Enable Proxy Arp on the Lan interface <= main magic.
For each peer entry, likely only allow it’s 192.168.0.x IP address
On the clients:
Give the wireguard clients IP Addresses 192.168.0.225-192.168.0.239
With allowed IP addresses on the client 192.168.0.0/24
And an appropriate DNS server (perhaps 192.168.0.224)
The remote clients, and local systems should now be able to connect to each other.
You may (or may not) want/need to make the wireguard interface a LAN interface.