The Colmi P8, which is older, long replaced by myriads of newer china watches and now hard to even find, was one of the last cheap smartwatches to be based on the nRF52832 microcontroller/SOC which had the advantage (for that purpose) of being both well documented and yet not locked down. The successor SOC, the nRF52840, already had a flash securing feature that (except for devices that wouldn't use it or that would have exploitable vulns) made it easy for the manufacturer to lock the device down and to prevent the install of alternative firmwares. Also about that time, cheaper chinese SOCs came out and cheapo china smartwatches switched to using those instead of nRF. Trouble being: most of those chinese SOCs for smartwatches, aside from probably also having the lockdown problem, don't have much in terms of openly accessible documentation or developer tools.

Consequently, pretty much all open source projects for cheapo china smartwatches apparently only support devices that are so old that you don't even find them anymore on aliexpress or other such shops.

I'd be interested to know for what currently easily available cheap (i.e. not in a much higher price category) china smart watches there is an open source alternative firmware that does not miss half of the features.