What partitions and filesystem should I use to install Debian, and then Proxmox on top of it?
Disk size is 512GB SSD in RAID1 and RAM 32GB. I want to be able to take snapshosts and backups.
I appreciate your help, as always. 
I think I made this question before but can’t find the answer
hmm… depends 
I’d say there is not much to be recommended, rather do whatever you prefer… I’d usually go with 1G ext3 for /boot, 32G swap, everything else will be / as ext4 with relatime,discard.
as you probably are aware proxmox uses /var/lib/vz as main storage point, however with just 512GB, I would not neccessarily separate that already.
your VMs will reside on that as diskimages, with virtio-scsi and trim you can keep them sparse and always take backups via the control panel anyway. at least good enough for me…