I seem to be unable to find a definitive answer, according to the wiki 19.07 is the latest version. After a few tests it seems that 19.07.7 is the latest one that actually works for me. Anything beyond that in the 19.07-train corrupts ubus (weird stuff). When I try to start uhttpd with e.g., 19.07.8 it gives me "Failed to connect to ubus".
Since 19 is no long maintained, I though I'd give it a shot to find out if 21+ would be an option for this (old) AP. I found a few topics that seem to hint that (some) NAND devices are no longer supported: Request Openwrt firmware for my all device - #2 by slh
Then there are people that actually did the upgrade, according to their posts Meraki MR18 Porting issue with ubiattatch
So I understand that instead of the ar71xx images, I need to use the ath79 ones. There were some issues a few months back, but they have been resolved (?). Where would I be able to find these? (Yes, I know this question has been asked before, just trying to combine all info regarding the MR18 ) The generic directory for ath79 does have MR12 and MR16 images though (https://downloads.openwrt.org/releases/21.02.3/targets/ath79/generic/)
Previous question regarding image location: Where to find ath79 (nand) release images? - #4 by thomasthep
If I need to compile myself, I'm more than happy to put some effort in it (with some guidance of course). And to top it off, if there's some kind of "monkey dance" necessary to pull it off, I will document it here for others.