Tax Returns

Ah, it's fear of the unknown...

It'd be an hour to find all the charity donations

You've just had that emailed to you. I guesstimate the rest (which aren't done through JustGiving). I've never been asked to justify the amount and I'd be reasonable confident I could if I ever was challenged on it.

I almost certainly underestimate but it certainly isn't worth the time and effort to get an exact answer. I'd probably spend a couple of hours to get me an extra £20 or so, so a guesstimate is good enough for me (and the HMRC).

another hour to work out how to register on their stupid system

It may take a week or so (elapsed time) to get registered as they have to send you a code in the post (maybe that's changed), but it really won't take long to register. You'll probably have to do it within the next few years anyway so may as well get it done now.

https://www.access.service.gov.uk/login/signin/creds?aoc=Y

find all the details from work

2 minutes to email your payroll dept and make it their problem. Ask for a copy of your 2021/22 P60 and P11D. It may take them time to respond but that doesn't make it take longer for yourself.

then an hour to go through the forms

Maybe first time, but you get faster each year.

then weeks of worrying I'd got something wrong and then more hours when my tax bill came in

There's no waiting. You get the calculation there and then and can modify it as many times as you like before final submission if it doesn't look right.