> I got a working A1200 AGA config (JIT enabled!)

Congratulations. I found some info on how to set the JIT, so you can compare:

https://morph.zone/modules/newbb_plus/viewtopic.php?forum=16&topic_id=12524&start=32
https://morph.zone/modules/newbb_plus/viewtopic.php?forum=3&topic_id=8762&start=53
https://morph.zone/modules/newbb_plus/viewtopic.php?forum=3&topic_id=8762&start=59

Bottom line: 'cpu_type' and 'cachesize' parameters seem to be the ones necessary, amended by reasonable values.