Something 'deeper' than Emacs, or am I looking for a unicorn?

The only editor that I can think of and matches Emacs on any of those points is Lem [1]. It has a few hypothetical advantages over Emacs:

- Lem is written in Common Lisp. It's less of a niche language than ELisp. You are therefore likely to find a larger library ecosystem.

- RMS didn't really like CL when he wrote Emacs. But CL is arguably a much better language today. For example, you don't need to worry about dynamic scoping.

- Lem is written entirely in CL, without any C core like Emacs does. That possibly makes Lem customizable to a deeper level than Emacs.

- Being a very young project, Lem is likely more optimized for multithreading compared to Emacs. Emacs multithreading is not up to expectations, perhaps due to legacy plugins and code.

Lem is not capable of competing with Emacs on any other points on account of the huge difference in their ages. Lem has much fewer extensions and a much smaller community. I don't know the project well enough to comment about their longetivity. But it's worth a look.

[1] https://lem-project.github.io/

Customizability: what are you trying to customize but you can’t?

Packages: https://melpa.org/#/

Lifespan: Emacs hails from the 70s and is actively maintained.

There are literally 100s of text editors, so it really is more a matter of taste and preferences. However, to the best of my knowledge, emacs with eLisp is unlikely to be eclipsed on any of your points.