They just sent me another reminder that they won't be reminding me anymore. I used to use certbot when I had an owncloud server. Then I switched to Nextcloud with a snap and it's sort of automagical. I didn't know it would do that so there's all kinds of left over bits and pieces of not-snap letsencrypt and certbot lying around that I must get rid of one day.

ISTR having done this previously by adding A records to the domain DNS to validate, but my personal domain registrar (GD, who else) has custom DNS servers set by my hosting provider, so I can't edit the DNS. Anyway it's been so long I'd have to read through it again, and it's probably all changed.