from the german thread, how to install angelfish w qt6 somewhat easy:
devel-su zypper ar -GF https://repo.sailfishos.org/obs/home:/piggz:/qt6apps/sailfish_latest_aarch64/ piggz-qt6-apps
devel-su zypper ar -GF https://repo.sailfishos.org/obs/home:/piggz:/qt6sb2/sailfish_latest_aarch64/ piggz-qt6sb2
devel-su zypper ref
devel-su zypper in angelfish
# also needed
devel-su zypper in qt-runner-qt6
# set dpi in runner to 280-320. else layout to big
# to deinstall
devel-su zypper rm -u angelfish qt-runner-qt6
# remove repos
devel-su zypper rr piggz-qt6-apps
devel-su zypper rr piggz-qt6sb2
# update package cache after remove, not really needed
devel-su zypper ref
edit;
jolla store remove entries added with above way, if you want to avoid this use ssu:
devel-su ssu ar "piggz-qt6-apps" "https://repo.sailfishos.org/obs/home:/piggz:/qt6apps/sailfish_latest_aarch64/"
devel-su ssu ar "piggz-qt6sb2" "https://repo.sailfishos.org/obs/home:/piggz:/qt6sb2/sailfish_latest_aarch64/"