By "backup" I meant something a bit different.
1) When I leave home with a backpack, it's always with 2 cameras, one with the RF 15-35, the other one with either the 100-500 or the 70-200
2) Longer journeys always mean at least 2 cameras, number 3 remains in the suitcase
3) Should R5 II need a repair, that's when R5 II number 2 takes over, as a real backup
Maybe I'm suffering from a severe form of neurosis/paranoia/anxiety +++++, but I'm "afraid" of staying in a beautiful landscape, or discovering a rare orchid only to find out that the one camera I took along fails...
For obvious reasons, namely weight, I originally planned my trip to Japan with one single camera and 3 lenses.
I thought it over...perhaps unfortunately.
Result: 3 bodies, 15-35, 24 TSE, 50, 70-200, 90 macro and 100-500. GAAASP! Third body in suitcase, but all lenses in backpack.
(My backpack has seen much much worse

)
The R8 certainly is a nice camera, very recommendable, not only as a backup. Problem: Different battery and a very low definition EVF.