I have finally ended up with binding both « thumbnails » folders through fstab adding those lines :
# .xcf thumbnails from gimp snap for any app
/home/user_1/.cache/thumbnails/ /home/user_1/snap/gimp/common/.cache/thumbnails/ none bind 0 0
/home/user_2/.cache/thumbnails/ /home/user_2/snap/gimp/common/.cache/thumbnails/ none bind 0 0
/home/user_3/.cache/thumbnails/ /home/user_3/snap/gimp/common/.cache/thumbnails/ none bind 0 0
This way, Gimp-snap actually writes its thumbnails there and then Nemo and Nautilus accordingly displays thumbnails for .xcf files.
gThumb though still shows empty frames for .xcf files - but my main concern was the file explorer.
Thanks to https://forum.ubuntu-fr.org/viewtopic.php?pid=22362050#p22362050