One way to combine the two directions in this thread: treat short-form as a discovery layer to test, and the stream as the depth layer. TikTok, IG Reels and FB Reels are built around recommendation feeds, so a new channel's clips have a route to non-followers there; whether that turns into actual discovery for your content is exactly what a small test would show. Twitch discovery is organized around live categories, which is a different starting point for a channel with no audience yet. That is why I'd pair method33's shorts idea with your stream plan instead of choosing between them: the shorts can be how future viewers find the stream.
It also gives you a cheap way to explore your actual topic question. Before committing to a stream format, cut 3-5 short clips on different candidate topics from your list and watch which one holds attention and pulls real questions in the comments. A week of that gives you signals worth investigating - retention and real questions per topic - from people reacting to the content itself rather than discussing topics in the abstract. It will not prove who would show up to a stream, but the topics that earn real questions are reasonable candidates for your first ones.