First of all I’d advise that alongside your development you keep a ‘community-building’ track running and reserve some of your time for that. Many of that need not take a lot of time, e.g. frequent masto updates on progress. Reacting to other toots and always name-drop FedeProxy with an url + hashtag to it. You may set up a gup.pe group for people to follow.
Simple things work too, e.g. stuck coding? Write a quick toot asking for help, and then do your regular solution search. Keep a blog to write a longer-form article now and then, and present Roadmap stuff. Might use Writefreely here.
I’d also invite you to get FedeProxy in #software and provide updates on federations / spec-related stuff. Partake in spec discussions, write FEP’s, join SocialCG meetups, etc.
Many people are quite interested in FedeProxy, what it might bring to development, and the new and interesting ways that AP is applied (i.e. beyond Microblogging). Play into that. Attract people in your forum.
Along the way many people will express needs and wishes that are far from your core concern (MVP). But maybe you can entice people to build related projects, ideally building on top of what you provide. This way they get a shared interest in contributing to the project.
And I’d focus some of my effort on the Gitea ForgeFed issue that @cjs has created. Get Gitea contributors to work independently on having an endpoint at their side, that interops with yours. In that light I would also consider hosting your code on Codeberg, which is dedicated to FOSS-only, and where Gitea team members are part of Codeberg team.