Ferawyn

I have tried opus, and indeed have had issues with players not supporting it. But that was years ago now, so I expect that's no longer an issue. Using ogg is mostly just inertia on my end.

Interesting idea, but I feel anyone who wants this already has the tools available.
Personally, I use Syncthing to synchronize a Media folder across my desktops, phones and tablets.
Media/Music contains my active music collection, mostly ogg conversions of the source flac files. I use .m3u/.m3u8 files as good old playlists, saved to the Media/Music root folder with relative paths. This allows players like AIMP on windows to play/edit those playlists, and players like GoneMAD on Android to play them without any kind of active internet connection.
There's also Media/Audiobooks, Media/Comics, Media/Movies, etc... Yes, they're subsets of the full collections on my NAS, but I've never seen that as a disadvantage.

I would suggest looking at Syncthing. It's not perfect by any stretch, but it works peer to peer, without any kind of central host, ip or domain name requirements. You simply install it on the client machines, and they work out how to talk to each other over any available networks.

Beware changing the casing on your files or directories though, Syncthing was made entirely case sensitive, which does not play nice with Windows.

One very nice feature is that it does have an android client (https://f-droid.org/en/packages/com.github.catfriend1.syncthingandroid/), and it supports full background syncing to your local storage on the phone. Great for syncing your photos, but also music. You add some mp3's on your desktop computer, and by the time you've put on your jacket they're on your phone ready to listen to without any 'service' getting in your way.

A more advanced tip; Get a VPS somewhere in the cloud with cheap storage, and have Syncthing on it listening on port 443. That will allow syncing in more restrictive corporate settings, which often don't allow connections to port 22000. And it gives you a 'cloud backup' of your important files in one go.

Proxmox is Debian. :-)
I do always suggest installing Debian first, and then installing Proxmox on top. This allows you to properly set up your disks, and networking as needed, as the Proxmox installer is a bit limited: https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_12_Bookworm
Once you have it up and running, have a look at the CT Templates. There's a whole set of pre-configured templates from TurnkeyLinux (again, debian+) that make it trivial to set up all kinds of services in lightweight LXC Containers.
For Home Assistant a VM is your best bet, as it makes setting up connectivity way easier than messing with docker networking. It also allows easy USB passthrough, for things like ZWave/Zigbee/Bluetooth adapters.

https://www.mailstore.com/en/products/mailstore-home/
Downloads email from most providers, allows for local (text) searches, and to upload it again to an imap provider.

For backing up your email from gmail or any other provider, check out MailStore Home edition: https://www.mailstore.com/en/products/mailstore-home/ It will grab everything in the account and store it locally, and then allow you to push it back onto any other imap service when necessary. Great for migrating your email, and keeping a just-in-case backup.

Email is the one thing I have stopped trying to do myself. It just has too many things that you absolutely need to keep updated. Have a look at Forward Email (https://forwardemail.net/en). They can hook up to pretty much any domain setup you already have, and do the heavy lifting for you.

If you're already going to the trouble of setting up ZFS for the two NVMe disks, I would suggest setting up a separate pool on the HDD as well. It will save you from monitoring two different filesystem types and give all the ZFS features, checksumming, compression, snapshots, etc... Do make sure your server has a decent chunk of memory through, as your VMs will be fighting the ARC for ram...

A proxmox root device uses barely any space. Mine usually sit around 12-14 Gb used. Writes are also negligible. DRAM less ssds are not a problem. I would suggest installing Debian first, so you can properly partition your root device before installing proxmox (https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_12_Bookworm). You'll have much more control over your disks and networking than using proxmox's own installer. Start with a 64Gb root partition, and leave the rest of the drive empty for future use/SLC cache.

Unless your VMs are somehow high volume data writers, like a proof-of-space-coin, I wouldn't worry about it. Homelab setups rarely reach anywhere near the kind of write endurance of ssd's.

Your VMs are not going to write to the root device, so it won't matter.

You won't notice the difference in performance of a filesystem on a rotating harddisk. Look for other useful features, like at-rest-encryption, and checksumming for bitrot protection.

I would use a filesystem with checksumming, rather than relying on any point-in-time check to monitor HDD's. Assume they will all fail eventually, because they will.

Security and bugfixes, after one or two rounds of testing by early adopters/key users. Preferably through some form of automatic updates.

New features and breaking changes, or anything that requires the end-user to pay attention, I'd say no more than 4 times a year, and using a non-automatic form of update. The hard thing is getting the user's attention on the changes, and not just clicking next and then having a broken or insecure installation.

Have a look at https://forwardemail.net/. It's a service that handles accepting (and optionally sending) email on your domain, and forwarding any received mail to other backend services, like a gmail account. All you need to do is set some DNS records, like MX and their servers will handle everything. It works fine with domains hosted on cloudflare, and has excellent howto's to get everything set up and running.

Edit: The great thing about this service, imho, is their guides. They don't just have a static howto, they template in your information into the exact string you need to copy/paste into the service provider's web interface. Want to encrypt your plaintext TXT records? There's a button for that on the guide. Want to learn how to get around a port 25 ISP block, they have a guide for that. Want to set up proper Send-As from Gmail using their SMTP server? There's a guide for that. :-)

No. USB is not designed to be reliable. It's designed to be plug and play. Don't plug and play with your data.