Nextcloud is a suite of client-server software for creating and using file hosting services. Nextcloud is free and open-source, which means that anyone is allowed to install and operate it on their own private server devices.
I used linuxserver's version
magick mogrify -monitor -format jpg *.HEIC
in a folder with the HEIC files.
sudo occ files:scan --all --verbose
(this does all users)
sudo occ maps:scan-photos -vvv
sudo docker exec -itu www-data nextcloud php occ upgrade
sudo docker exec -itu www-data nextcloud php occ maintenance:mode --off