Recently I have been also thinking about TeX's internals, also because I feel the pain points you describe in at https://www.finl.xyz/. I have too thought about experimenting with TeX rewrites/alternatives and particularly in Rust.

I don't know if you are aware, but finl has been also submitted to Lobsters (https://lobste.rs/s/udu5oe/finl_is_not_latex_reinventing_lat...). One of the comments mentions SILE (https://sile-typesetter.org/). I looked into it (https://www.youtube.com/watch?v=5BIP_N9qQm4 was nice introduction, although I don't know how up to date it is). It seems to me that SILE replaces many pain points in TeX, while being somewhat close to LuaTeX (and TeX in general), due to use of similiar components (Lua, alogorithms, hyphenation patters, libraries). It even supports "LaTeX" like input syntax, but frankly I don't think that is the part of LaTeX I would keep :).

Can you share your opinion on SILE?