Hello, I have some experience using Debian in NAS, but none with TrueNAS.

Before anything: BACKUP !

If possible, make a full copy of your pool onto external drives, or another NAS or anything else. If it is not possible to get enough spare storage soace, then at least backup the things your really care about (personal photos, important projects, password database). Just make sure you have a valid backup in case things go terribly wrong ! I am sure everything will go well, but this will give you additional peace of mind.

Setup Debian for NAS use

There are a few things I can think of, many might be obvious, anyway:

1. Install and configure network file sharing protocols: Samba (files sharing compatible with Windows, Linux, Android and others), NFS (more like network drive)
2. Install: S.M.A.R.T monitoring tools
3. Choose and configure filesystem. I have been using BTRFS, but since your pool is ZFS and you probably don't want to format and start from nothing, I think you are already set on that one. I believe this is extra configuration as it doesn't come by default on Debian.

More advanced things:

1. Setup SSH for remote connection with the terminal
2. Install htop or btop for system monitoring in the terminal
3. wireguard is a very nice VPN, it's easy to configure on all platforms in order to access your NAS from outside your home

Power optimization

1. Enable C-stats in the BIOS (warning in case your CPU is a 1st gen Ryzen, do not activate)
2. powertop is an utility to optimize power saving settings (I've not bothered with this until now)
3. hdparm is an utility to manage and configure hard drives, you can use this to configure automatic spin-down after some time of inactivity, this is a bit tricky though.

Dashboards, UI, frontends

1. OpenMediaVault is the first one that comes to my mind. It's actually a Linux distro based on Debian with a web interface that allows you to do all the NAS relevant confirmations from the Webbrowser. It can also be installed on top of an existing Debian install. I have used it a long time ago.
2. CasaOS, similar in concept, I have not used it.
3. Another simple option to get started, get a monitor, keyboard and mouse. Install Debian with full desktop environment and configure everything in person. You can always go headless at a later time

Docker

You definitely want to install docker to run most of your services. Please, also add your local user to the docker group to not have to run everything as root. Useful services I use:

• Portainer: manage containers with a web interface
• watchtower: Auto-Update other docker containers
• Jellyfin: media Manager and player (similar to Plex)
• *arr, transmission, sabnzbd: sail the high seas
• gluetun: route containers traffic through a VPN
• caddy: reverse proxy, to access your container with sub-domain name, example: jellyfin.mycoolnas.net
• vaultwarden: redistribution of the Bitwarden password manager for self-loading with the premium features available.

Graphics cards

It's been a long time I didn't have to deal with NVidia. Debian comes by default with the nouveau open source driver, which works but may not give the best performance. I don't know if it impacts transcoding performance. I suppose it doesn't give your the NVENC codecs. Anyway, you can install the NVidia proprietary drivers and should be able to transcode.

Conclusion

Debian is a solid option for a NAS, it's been serving me well for many years. It is set and forget. However. It takes time to setup and the terminal is going to be your main configuration tool unless you go for OMV or another distro specialty made for NAS.

Your main source of information shall be the Debian Wiki. You will find step-by-step guides to install most of the things mentioned above. The Arch wiki is also a good resource, keep in mind that some files may have different locations and package different names across Linux distributions, but configuration should be similar.

Best of luck my friend