There are the nordic nrf52xxx based btle smartwatches for <$30, search 'smartwatch P8' for example.
The firmware can be replaced with your own.
The battery won't last a year though.
Any good resources for replacing the firmware and other hacks?
Try wasp-os https://github.com/daniel-thompson/wasp-os
I have a colmi P8 and it was easy to get wasp-os uploaded and now have micropython available with direct connection into the watch!
Battery life is really only 24-48hrs if you don’t do anything heavy.
Sometimes I wonder how more useful it is than my apple watch.