How to start a local tech meetup?

Personally I'd look at sites like meetup.com to see if there's already groups in your area. In my region the level of event activity is still lower than normal, but it does seem to be coming back as in person activities are becoming more routine.

I feel like I get enough tech from my day job, but from what I've seen skimming through various group listings is the formula for tech meetups is some sort of set of presentations and discussion period afterwards scheduled regularly about once or twice a month. Providing information as to what's being presented entices people as they can decide "would this be interesting" prior to attending a group. Depending on how you treat such events it can be viewed as standard professional networking, but is that a bad thing? It sounds like you're interested in people working on side projects and exploring new ideas and those people are likely in or adjacent to your own profession any how.

If you're not interested in the in person aspect of things, there's loads of online communities out there for various niches, hobbies, projects, open source endeavors, etc. They're spread all over the place, from forums, to discord, to IRC, to in person makerspaces/hackerspaces, to all sorts of corners of the net. Either which way things will be hit or miss until you find an environment that meshes with your own goals.

Consider visiting your local makerspace/hackerspace. You'll find a large variety of interests and projects, and people interested in actually making stuff, not just talking about it.

I belonged to Pumping Station One in Chicago for a few years, it was a hoot!