We should definitely now start with a model which runs ok - but slow - with one of the existing forward solvers (bdf or rk45)… and then switch to adjoint.
Given that we struggle that much it would also make sense to go back to the SBC simulation I wrote a while ago. That simulation showed that the Adjoint ODE implementation at the time was OK.
For now, I do think that the changes made in the branch are an improvement and I’d hope that @stevebronder finds the time to review them so that it is included in 2.28.1 for now. It seems we need to investigate further apparently.