And I find that forcing a breakneck pace is also bad for an rp because it forces some people out immediately, and causes burnout far quicker, potentially driving more people away and not necessarily offering encouragement to new joiners.

This is the main disadvantage of the forum rp. Unlike meeting at a house where everyone sets aside one night a week. This is subject to peoples daily lives. I'm happy that you are able to post every single day if required. But not everyone can.

I would say 3-4 days myself. But ultimately this is the GM's decision.