Every couple of months sounds good. I think it’s good not to have critique on the comps – if you post and you’re not looking for crit, then getting someone’s comments (however well intentioned) can be bruising if you’re not used to it.
Failing that, do we want/need it to always be a comp? There are other writing sites that post a topic/prompt and simply invite a word limited piece… No pressure to find a judge, no judgement of the piece itself, just the satisfaction of reading what’s there and posting something if you’re inspired. That’s not to say there isn’t room for comps as well, but we can share themed posts without the need for them to be competitive, IYSWIM?