after getting help here building a site-to-site VPN with Wireguard, I kind of accomplished that, but it's still running not quite as I wanted...
The setup is: two sites with an Internet Router each (Fritzbox 7490 / 7590) and on each site a 2nd router with OpenWRT for the VPN (Fritzbox 4040).
After setting it up like being told here OpenWrt & Wireguard Site-to-Site it basically works, I can reach the Internet Router and OpenWRT router on the respective other sites, plus I can access machines via Remote Access (RDP and VNC) over the Wireguard VPN. So far so good.
What does not work however is any kind of networking as in a local LAN, i.e. I can't ping any (Windows-) machines on the other site (pinging the routers works, though!) and I can't access them either with
PCs running Windows 7 and Windows 10 doesn't seem to make a difference, there is just no way to do any file-/folder sharing over the Wireguard VPN.
I have a hunch it might have something to do with the OpenWRT Firewall, but really got no expertise how to fix it.