Teams works fine on GrapheneOS.

I recall Google Maps working briefly before I fired it in favor of buying a dedicated hardware map solution.

If they do work, which De-Googled OS should I go for?

Depends on your needs, I suppose.

All the options you've listed are an improvement over stock.

I had an old spare pixel to try out GrapheneOS, and it was faster than my newer phone, so I started buying GrapheneOS-ready hardware from then on.

I put LineageOS on some other hardware, and I miss some privacy features, but it's still pretty nice.

Lastly, if I install another OS and regret it, is it possible to install the stock OS again?

Yes. There's instructions to take a back up, at the start of the process.

My plan was to fall back to eOS or LineageOS if I hated GrapheneOS, but it's been lovely.

Edit: Also, what happens after Google's "Unverified App" decision has passed, will this affect these alternative OS?

That's all part of Google's app ecosystem. Install F-Droid or Droidify, and you'll be immune to all that. Aurora may work as well, but usual disclaimer for Aurora - it relies on Google and might break at any time.

GrapheneOS has made public statements GrapheneOS itself is unaffected.