Hey, welcome! This is a great line of thinking, one I am also definitely interested in. I agree that markdown is not the ideal interop format for anything beyond the basics (it doesn’t even have a mechanism for encoding block position, i.e. indent or columns). I do think it is one possible format for a base level of interchange. But we need more.
While I think Anytype may have to maintain some unique blocks and functions due to its particular architecture, at the very least I think it should support Block Protocol. I haven’t seen a lot of support for it elsewhere yet, but such things can be a bit chicken-and-egg: someone needs to start supporting it even before others do, and that initial support will help convince others. Anytype could be one team forging that path.
It would also be interesting to know if there are other efforts like Block Protocol. I’m not aware of them, but with all the block-based apps happening lately, it seems quite possible others are thinking and working along similar lines for interop.