emacs-async VS hors

That is probably the simplest option possible; but if you need non-blocking evaluation, async package is definitely a better option.

You already can. Using https://github.com/jwiegley/emacs-async or https://github.com/chuntaro/emacs-promise.

IMO the best option currently is async by Wiegley. It will manage Emacs instances for you and do all the low-level synchronization and messaging for you, so you can work in higher level abstractions as if you are working with threads.

Have you checked the async package?

That is quite normal thing to do. Have you not seen Emacs Async? Take, a look, it is a useful thing. Or Emacs Request. Since Emacs does not have proper thread scheduler, that is the best next thing you can do.

There are ways to avoid this, have you tried e.g. https://github.com/jwiegley/emacs-async ?

As a note about the third video, and searching for backlinks; the volume, when you get there, might be a slow-down when you work with many small files, like searching for backlinks. Each note means a separate file access, search process, etc. It is much more efficient for computers to read one big file, then many small files, and then just use Emacs to search in that file. If you are a developer of Denote, you might wish to look at asynchronous processes or perhaps use Wigleys Async package to search for backlinks asynchronously.

Async process can do that. Have you checked async library by Wiegley? You can use another emacs process as a sort of clean interpreter thread similar to javascript workers.

That "99% of standard usage" is the kicker, isn't it? Those greybeards who always opposed multithreading since long ago tend to say that the remaining 1% of use cases is best done in an external process, ideally not even written in Emacs Lisp, so that the rest of the open source community can benefit, like the GNU Global you mention. I suppose if you still want that program to be written with Emacs Lisp, you could use async.el (https://github.com/jwiegley/emacs-async/) and there's finally an use-case for the threads: it'll be relatively safe to run those 16 threads only in the external Emacs-process.