Hi
You musn’t forget a few Windows “Standards” you’ld need to implement, also not yet standard on Neth…
Windows Profiles Directory / Share for clients that need / require “roving profiles”. This works well.
There is a good dok here:
→ Don’t forget to include this folder in NethServer’s own backup!
You can also rsync the contents over from the old AD.
For file level Backup, both win and Neth, I use my own rsync script (Running on Neth!). This places the files in a Share “Backups” on the NAS, with subfolders (1_Monday, 2_Tuesday, 3_Wednesday…). I keep 7 generations, users have read only permissions to the folder. This makes this safe from cryptolockers.
My NAS are never in AD, and the local admin has a different Password from the AD / Neth admin, also a protection from Cryptolockers.
Actually, you’re specifically lucky the present server is Win2008R2. This version is fully supported by Samba - all functions! So NethServer would be seen as “equal” by the old AD…
For a 2016 or 2019 AD the mentionned route with join - demote - promote would be quite risky!
RSAT and Stephdl’s PHPLDAPadmin are priceless tools when it comes to AD administration.
I’d setup a dedicated Win10 as PC-Admin10.
As to Proxmox and TrueNAS: both companies behind them are among the largest contributers to the fantastic OpenZFS project. Proxmox also for CEPH, one of my next projects.
OK, food’s up. I’ll be back shortly… 
My two cents
Andy
This could also help:
https://wiki.nethserver.org/doku.php?id=userguide:nethserver_and_proxmox#proxmox_hp_rok_server_licenses_dell_equivalent
To read it out of the present server(s) (To test AD, etc…) boot with Linux, eg SystemRescueCD…