Thanks for the replies everyone. I spent AGES trying to figure out something that might work, esp considering that I typically embark on this kind of thing expecting to be super organised and then muddle things up a bit.
I tried Narrative for doing an intermediate selection / cull, but though I can appreciate how it might be useful for people adding new stuff regularly, it just seemed like another layer of unnecessary complexity that would probably be more of a hindrance than a help.
I also gave up on the cloud version of Lightroom as I could see I'd end up paying loads if I tried to store everything.
What I finally figured out seems to be a pretty decent system. I've put all the images and videos into a folder on Dropbox separated into year folders, and then into separate folders for the day / trip / event. I've then used Lightroom Classic to create a catalogue, with the catalogue files in a Lightroom folder in that same photo / video folder. This means that I can use LR on both my laptop and my desktop to open and work on stuff, and the same catalogue files get updated whichever one I use. The only thing I have to remember is that sometimes it takes a little while for the catalogue file to sync across Dropbox, but seeing as I'd usually be using one computer or the other, I don't think this is going to be an issue.
I was going to look at using another cloud backup on top, but Dropbox (esp Plus, which I'm currently using) seems to be more geared up for backup as well as more general cloud access / sharing these days. (Both computers now also individually sync to Dropbox Backup now.) But I think I'll also have a physical drive attached to the desktop to backup the whole of my Dropbox once a week or something.
Hopefully this should let me keep everything, whilst also maintaining a decent amount of organisation. It also means I can upload everything to the final location whilst I'm away, vs storying it on a drive on my laptop and uploading it all when I get back. Fingers crossed.