About ten years ago, when smartphones just started appearing, the forum did not have a mobile version, and there are various 3rd party clients on the App Store or Android Market.
Later on, one of the largest 3rd party client was blocked, because of they hammering the forum's servers too hard,. Or something about caching and stealing ad revenue.
Then a couple years later, in 2017, the 3rd party client's devs launched its own forum reusing the client's name. It exploded in popularity and quickly took over as the most popular message board among the youth.
The old forum now has a sort of boomer or mentally ill stigma to it.
I hope to see Apollo go down this route.
Oh, and I think both forums in the story did not monetize as hard as reddit going to paid awards and memberships.
One more thought: Keep the Apollo UI or whatever thing the users are most familiar with. Most of them do not care if it is fediverse or open source or backed by web-scale k8s, they only want it to just work (tm) good enough to post things on it. Eat the lunch you prepared yourself.
Twitter used to have a vibrant ecosystem of clients, most of them working better than the official app both on web and mobile. Twitter was able to kill their third party app ecosystem with their paid API changes and lived to thrive as a company till recently.
The reality is, you need a good cohort of content and users to move who can sustain the content, moderation and discussion. Just moving Apollo users doesn't ensure that. There are other good third party clients like Relay etc as well