The main reason M3 took a long time isn't related to m3 itself, but rather that the asahi project took on a ton of tech debt to get M1/M2 working. M3 wasn't too difficult, but before taking on the additional tech debt, the Asahi team focused on getting all of their changes upstreamed to the linux kernel.
Prognosis is then that work for m4/m5 should be relatively straight line now that refactoring is done?
The main developer was also the target of a harassment campaign from a place that has pushed other targets to straight up suicide. That took almost all of their energy for the last year and they ended up quitting.