Just to come to closure, the GIMP3 issues are not with the snap version or App Center (beyond the missing dependency of the “openvino toolkit”.
The gory details are here: https://github.com/intel/openvino-ai-plugins-gimp/issues/207#issuecomment-3353655278
The short answer is that the plug-in is a memory pig and tries to allocate ~15GB of RAM for a 6 Mpixel image, the i5 system I tried it on only has 8GB. Running it on a much smaller ~600x600 image succeeds and it does do a better job than just up-scaling the image. But without a GPU I suspect it will be way to slow to be very useful.
Decent error reporting in the GIMP3 plug-ins would have eliminated the issue entirely.