I don’t think it is functionally too restrictive, but I feel like it has three problems — one which probably generalizes to other sites, another which is kind of specific to my situation, and third which I also think generalizes.
The first is simply that it’s additional confusion — new users need to learn about trust levels and permissions at some point, but now they might need to learn about it earlier, which goes against my goal of a more simple experience. In some cases, Feature request: "redirect" "New Topic" button to a valid category when user doesn't have permission to post somewhere might help.
The second is that while Discourse is (relatively) new to our community, the community itself goes back 20 years. I had some early frustration with people trying the site in good faith, and then getting really annoyed at the restrictions on links and images. I therefore intend (once we have our group sync thing working) to bump most new-to-discourse-not-new-to-Fedora people to t1 immediately. This would make it possible to restrict some of the groups meant for more-involved contributors, but then ironically leaves us back to the beginning — people new to the site have more choices than necessary.
Third, as an admin who can post anywhere, I’d like to reduce my own cognitive load when selecting a group. I can easily see this being the case for many TL3+ users and mods … or even TL2 users. Unlocking capabilities and complexities as users are more familiar is a great general design pattern… but for some things, advanced users benefit from simplicity as well.