I am soon going to do a complete reinstall of a dietpi server and when I do that, I would like to get VPN setup with portforwarding.
Right now I am using ProtonVPN, but it is not possible to port forward on linux with it (that I know off), so I would like to ask you what service is the best and easiest setup for dietpi at the moment?
Can you give me a recommendation (and possible guide) for a service.
Thank you 
PIA supports port forwarding and is directly supported by dietpi-vpn: https://www.vpnranks.com/best-vpn/port-forwarding/
The others from this list can be added as well with their OVPN config file.
However, probably it’s better to use their WireGuard based setup instead of using the OpenVPN-based dietpi-vpn.
But better to wait for a reply from someone who actually uses one of the providers with port forwarding support and can share experience from first hand.
I guess, setting up some split tunnel could do as well?
Not if the incoming connection shall be tunnelled through the VPN server/provider.
true, depends what scenario you like to implement.