@Silver I would vote for bringing in beta testers early, but with the caveat that you limit the number of people who have access. Perhaps stage it like you did with the app, bringing in a few more devs every week or something like that. I wouldn’t want the Obsidian devs to get overwhelmed with feedback and requests and plugins. You could have a short application form where the prospective dev describes the first plugin they want to create, and then you can select for a variety of plugin types. At this early stage, I might suggest the process be more about hammering out the API and less about creating finished, polished plugins for deployment to the wider community.
Re: @anshbansal and auto-updating, WordPress might be a good model. Users have to manually update all plugins, and plugins have to report what minimum version of the app they are compatible with. WordPress also forces a minimum PHP version, which may not be necessary with JavaScript/TypeScript and compilers.
Disabling all plugins = maintenance mode
Second @ksandvik on building the API in TypeScript. Have you considered what kind of documentation tooling you are going to use?