X11 (no forwarding, it’s really meant to be that way - originally at least) would be the simplest and lightest. If you’re on a private lan you might also turn off cryptography… the powerpc cpu probably either has no crypto acceleration or acceleration for old cyphers.

Other solutions might work… guacamole could work but would use a whole other desktop (whereas x11 would still have some resemblance of integration).

If you’re okay with remoting the whole desktop, xrdp and vnc are also options…