I wrote a HowTo about how to upgrade to Nextcloud 26 on Debian Bullseye, using either Ondrej’s PHP repository or upgrading the system to Bookworm: HowTo: Upgrade to Nextcloud 26 on Debian Bullseye
Everyone is invited to give feedback, in the hope we have something helpful to give admins at hand who have not so much experience. I aim to collect and update it with solutions for possible issues and maintain it as (community) support thread for the process as well.