Storage and considerations for my disks

Converted my old gaming PC into a server to be used for self hosting. Proxmox up and running. But I feel like I need some advice on storage and priorities if I'm going to buy upgrades. My disks now:

Disk 1: SATA SSD 250GB (Proxmox OS disk and lvm-thin partition)

Disk 2: HDD 1 TB

Disk 3: NVMe 2 TB

(Not installed, spare Disk 4: HDD 2 TB)

Future plan is to two-parts

1. Have a ZFS pool with 3-4 Disks (RAID-Z or ) to store various media that is not super critical if lost (data pulled from web)

2. A seperate NAS to hold hold my own and family private cloud storage, think Seafile or some storage solution with various client support (compute might be on proxmox). This I need to think serious backup.

Questions:

1. Something immediate I should do with the OS disk, like mirroring so that server doesn't die if fault occurs on OS disk (or have I misunderstood something here?) Or is the answer, just add another proxmox server to get more redundancy, since other common-mode failures..

2. How should I share a disk or pool for several VMs or LXCs to read and write to? I have read about bind mounts, but also virtual NAS (NFS share) any reason to choose one over the other? I kind of like the virtual NAS idea in case I later migrate the data storage to a separate NAS..

3. I want to get started with what I have now, but with minimal friction when expanding system. Anything I should avoid doing, any filesystem I should avoid? Am I correct in assuming that I need to migrate data to external disk and then back if I want to put say disk 4 into a RAID setup later while just using it as a single disk for now?

4. Can I start a pool with Disk 2 HDD and Disk 4, striped, and then expand and change the RAID setup later?

5. Any good usecases for the NVMe disk, as I'm just planning for HDDs to hold media and stuff? Also, I assume combining SSDs and HDDs are bad in a pool?!

Sorry, that was a lot of questions but any replies are welcomed :-D