Everybody on HN always gets huffy and prima-donna-ish about the idea of a take home assignment. So I'll try to provide a different perspective.

For some people, like me, at least in theory, I would love it, because it's much easier for me to shine than when put on the spot in an interview.

I like to talk in an interview, but I'm no good at retrieving technical knowledge in real time.

Unfortunately, in real life, the one hiring process I went through that required a take-home assignment, I realized at the end, I wasted my time doing it, because ultimately it was not deciding the outcome, but a regular in person coding session was. I assume this is because you never know how clever people can be in cheating remotely, so they can't really trust the applicant did the project.

Now, the way you list your steps in hiring suggests the take-home assignment (and presentation) is the final qualification, not the first screen. Which should be good, if it really is your intention...except...

I see a contradiction in your post. You want a "SWE", a "senior", and someone who already has a "deep understanding" of the underlying platform.

The whole point, for an applicant, of doing a take home assignment is that they can demonstrate their intelligence rather than knowledge or ability to think quickly on their feet. If you don't want that sort of person, and it seems you don't, then who do you expect to apply?

I'm approaching the same conclusion as other people, but I'm trying to avoid explaining it in terms of seniors being too good to do take-home assignments.

Plenty of people in tech hate interviewing, don't have good networks, and some of them have a lot of experience. Maybe not with your tech, and maybe not with a SWE title though. You'd call them juniors, maybe. Or, you'd treat someone who's never been a SWE per se like they didn't know what a computer was.

Something I have never understood was why hiring seems to almost always be following a paradigm of filtering the pool of candidates for a very specific imaginary perfect hire, rather than taking the best candidate available independent of the job and figuring out how to utilize them.

And an afterthought: your use of "rubbish" suggests you are in the UK, while "fully remote" suggests you are seeking global candidates, and maybe your idea of a reasonable salary in the UK for a "senior" nullifies your willingness to hire from anywhere. Obviously there are lower wage areas, but maybe the people you are looking for are mostly in higher wage areas.

Also, just wondering - suppose a candidate takes much longer than 4 hrs or whatever you consider normal. Is that a problem in your mind? Are you screening for that? Are you happy hiring them as long as they don't brag about how long it took, and do a good presentation?