And I think that this is a problem.
Nothing you say is wrong. But, as you say, it will have scaling issues. It doesn’t have a scaling problem now. Once there is enough activity that the forum becomes illegible, migrating to separate ones is very straight-forward.
My point is that splitting this forum into multiple ones attempts at solving a problem that doesn’t exist, and has major drawbacks. Only once the benefits of this split outweigh the drawbacks the splitting will be a good idea.