How do I know which home automation project I should invest my time into? I'm aware of at least 3 FOSS ones.

Home Assistant is the clear winner, in my mind. It's a very robust project (four full-time devs, plus a huge community), very actively developed (https://www.home-assistant.io/blog/categories/release-notes/), and has a very healthy and helpful support community.

They do very similar things, usually even identical. Get the one that does the things you like (i.e. that's ready with very little configuration to do the things you would like for it to do). Maybe look at the community, see if the help given is given freely and positively. That will impact your enjoyment of the software and I feel like that is the most important factor.

There are a lot. Most robust community appears to be Home Assistant. It’s not very point and click friendly yet, though. If that matters to you.

Here's current discussion of Home Assistant: https://news.ycombinator.com/item?id=21665125

Go by community. Home assistant is absolutely fantastic.

Home Assistant is the best one by a long shot. Largest community, most integrations, etc.

Just one more comment for the pile of 5+ that already replied.