hi @Dom_Dom connection from (Wireguard) VPN should be more or less the same as you have internally from your LAN.
I still don’t understand how you installed your Nextcloud instance. Is it running direct on Raspi, is it docker or any other virtualisation technology? How do you access you Nextcloud from your internal LAN (note the IP).
As next step I would double check which IP you see when you contact http://personaladdress.duckdns.org from Wireguard VPN - is it you public IP, is in internal of the Raspi (or you Nextcloud). If the IP is internal - it is expected to work exactly the same as within your internal LAN.
If the address resolves to the public IP you may hit rebind protection or “internal LAN IPv6 issue” - you find lot of examples searching this forum for rebind issues like here or here some of the posts are in German but Google translator or deepl are really good today 