I am using the docker install and managing it using docker compose, I scripted an automatic backup into the upgrade process, so if something goes wrong I can rapidly downgrade no matter what the new version did to the db.
Docker stores its files in /var, so that needs a bunch of space, I am using 24GB but it has gone up to around 40, the more containers you run the more space you need, 64GB should be minimum. More space also allows you to keep prior image versions for a longer period of time without purging.
for /boot I suggest 2GB if you are regularly upgrading kernels or playing with multiple kernel versions, otherwise 1GB, I use BTRFS and NFS so I need 5.11 or later for certain performance and reliability features, but also keep at least one prior kernel release AND the distro LTS kernels (5.4 right now).
for / I am using 20GB, so 64GB gives plenty of room to grow
for /home I am using tons of space on the computer with interactive login, 36GB. This is where I do code compilation and includes various SDKs and dev environments, browser cache, “snaps”, downloads, etc. If you are using the machine only for remote terminal work or running dev work in docker, then you probably do not need a /home partition, or you can keep it small like 16GB. I have more than 300000 files in there, so the separate partition helps the filesystem management.
Using Proxmox requires precise and thorough evaluation of the requirements for the underlying virtual machines, you could probably run nextcloud in docker instead, you will get better performance and it will be easier to manage. I would suggest an additional SSD in the system for daily backups of your important data, combined with a backup to a secondary on-site system (the pi if you can fix the freezing?), and have that manage offsite backups.
Estimate that for a docker host you should allocate at least 170GB of space