RE the last point: if the candidate mentions side projects, I'll typically probe there. It shows me they have a genuine curiosity and probably a motor. Anyone with a motor and is a good cultural fit will succeed quickly.

Side projects may show curiosity ... but I definitely don't gate-keep if you don't have any side projects

Side projects outside their "lane" are far more interesting to me (eg you're a Rust developer and on the side breed cornish game hens, for example) - demonstrating that you're "an and"[0] shows you can draw from multiple fields to find solutions

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[0] https://news.ycombinator.com/item?id=30855775

Fair point, but it is a good indicator.

You described a junior.

A good junior will at some point gather experience and become a good senior. Which of those aspects would you say change when they become a senior? Otherwise I don't see why this doesn't apply to seniors as well, a bad junior is unlikely to learn a lot while a good junior will make the most of their experience and become a great senior, so looking for the same things in seniors as you do juniors makes sense.

Checking for "senior" skills doesn't make sense however, everyone learns different things from their experience, if you gatekeep based on things you learned as a senior then you will miss out on a lot of experience in things you didn't experience yourself, it is much better to just test for good core attributes and value their years of experience based on those, as other than that there is no way to properly evaluate high level senior level skills unless they have the exact same skills as you.

Nope - I described everyone I've ever interviewed who's been any good, from intern to senior architect