Belts are always an option. It may also be the case that putting a seat in the middle (with the belt, in this case) means the side seats can't align with the ISOfix/LATCH points and need to use the belt. Also, ISOfix/LATCH is only rated for seat+kid under 65 lbs, which doesn't last very long with some seat/kid combinations, and then you're on the belt anyway.
In our Honda Fit, the middle was only place a rear-facing seat could go with two adults in the front, with the seat sticking between/over the shoulders of the front seats, so we seatbelt installed there. With the rear-facing seat in place, the booster needed an extra 1/2" to the side so didn't align with the LATCH points and had to be belt only (LATCH on the booster is only to keep it from moving around when empty; the child uses the belt).
When I moved our two child seats into the ID.4 this week (yay!), I immediately noticed the rear-facing seat in the center would be a problem. The rear seats are higher than the front, much more so than in the Fit, so when I tried the same arrangement the view out the back was completely obscured. Fortunately, even with the driver's seat set for me at 6'2", the rear-facing Clek Foonf can go behind me without (quite) contacting my headrest. My slim mother was able to sit between them, reasonably comfortably.
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The transition from seat back to door opening seems shallower than some, which may help in squeezing out those extra few mm. Need better light for that pic. I've got a cover on the way for the back seat (toddler sneakers + lunar gray upholstery is risky!) so will have the child seats out and can take some measurements, if it's helpful.