Thank you a lot for the detailed overview and status, @dcaliste .
For people desperate enough about this, email-oauth2-proxy may be a workaround. It appears to be similar to hydroxide (a “bridge” application for protonmail), which does work with Jolla’s General Email account type as discussed here.
The email-oauth2-proxy README alone is very interesting and provides a lot of insight.
Of course, getting this to run (which I have not tried) looks like considerable manual work.