@jdp23 Hopefully this hasn't already been said but social.coop are making all their collective decisions using Loomio.com. Could ask some people from there their experiences?
Discourse.org is made for this as well. https://karrot.world is made for organising events so not quite what you were looking for, but still interesting.
https://engagementhub.com.au/participation-tools/ and https://www.socialpinpoint.com/ are also engagement platforms.