Just wanted to add my 2c about shipping an FSE MVP in WP5.8 :)
True, not everything will be possible in the MVP. Users will want to do things that will not be possible out of the box in the initial version. But that's what an MVP is.
It doesn't have to be able to do everything under the sun, simply set the foundation, something we can work on and iterate as we go.

Just like the block-editor when it was first included in core didn't have all the blocks and options it has now, it keeps evolving and improving. But that evolution would not have been possible if it was not merged in core...

FSE has the benefit of not breaking anything. Nothing changes for existing users on existing themes, they won't even know it's there unless they use a theme specifically built for Full Site Editing.
Right now the APIs are solid, the foundation is all there, and using a carefully crafted FSE theme can result in amazing results.
Is it perfect? Nope! But what is?
Does it work? Yes.
Can I build a site with it? Yes!

Using TT1-blocks as a measure of "readiness" is wrong IMO. We use it because at this point in time it's the de-facto, "official" FSE theme, but it's not an indication of what FSE is capable of doing, or what an FSE site will look like. It's just a testing tool, built to resemble an opinionated core theme.