Linux

I recommend you Aurora. It is basically Bazzite, which you already suggested, but without gaming stuff.

Why do I recommend you that?

• The auto updates are amazing. Don't disable them. It isn't like on Windows, where it just randomly says "Updating, please don't shut down your PC" midst working. They get just staged, so they are only applied passively on the next boot. You don't notice them.
• Rollbacks: If an update introduced breaking bugs or whatever, you can just keep holding the space bar while booting, and you can select the image from yesterday. Everything is left how it was yesterday. You probably never have to use that feature anyway, the system is super reliable.
• The release schedule. This one is the most important aspect for your case. uBlue (Bazzite, Aurora, Bluefin, etc.) started offering different variants/ tags if the same image. There's now a GTS variant around, which uses the last big release of Fedora, which is still kept up to date maintenance wise. So, you are always half a year behind in terms of new features, but it has been tested for half a year more than regular Fedora or the other images. When you choose the more conservative GTS variant, you'll get way fewer surprises.

After installation, you can hop into the terminal and use the ujust rebase-helper, where you can select which image variant you want to have

• latest: synchronous with Fedora
• stable (default): features are two weeks behind
• gts: already said, last release, but still secure and more polished.

I think it is the perfect balance for you between "Debian is too stale" and "Fedora and many other distros change too often".

is support a factor in your decision? if so i would go with opensuse since it has options to let get enterprise support should you end up needing it. (anecdotally: redhat & canonical's support are better; ESPECIALLY landscape since you mentioned nvidia & proprietary codecs, but it is very pricey)

I use Debian for anything that matters. The release cadence means that stuff just works and keeps working. You cannot beat the documentation and I've been using it for 25 years.

I'm not touching anything Redhat / Fedora with a barge pole.

Not sure what the attraction to Mint is.

Never used OpenSUSE.

I can vouch for Linux Mint / LMDE; their pre-installed software and defaults seem very sensible and I need far less set-up, fixing and fiddling (esp. with NVidea hardware; the open-source driver refused to make anything run on GPU with my Asus ROG Strix GTX 970) then on bare-bones Debian or Ubuntu LTS.
All four mentioned here have very stable and safe release schedules.

Bazzite's defaults help a lot with gaming (and that stupid NVidea driver) and the initial welcome-screen helps you install the Steam, Lutris, OBS, etc. you want and leave out anything you don't. It's actually helpful, really!
I do want to add Bazzite's team seems to have only one person who can sign releases, and they did misplace a key at least once leading to nobody receiving updates until they replaced the key in their installation.
Their team management does not seem the best; assuming this was a one-off thing Bazzite can still be a great, stable choice.

Does LMDE do wayland? If not, I wouldn't bother.

Bazzite or some other Fedora variant. Nobara has some tweaks for nVidia, full codec support, and specifically supports programs like daVinci Resolve, KDenlive and OBS Studio in the Welcome screen. Not an immutable distro like Bazzite though, which depending on how you like to install stuff, might not be a bad thing.

MX (Debian + Nvidia + tools to make use easier).

Debian: Release cadance seems too slow for my preference.

Install OBS and other software from flatpak

Flatpak solves some of these. It would allow newer software on Debian. They’re packaged with codecs, so you don’t need to bother with Packman on Tumbleweed.

I like Debian. To save you the misery, though, you should probably just use the OBS Flatpak with it. I used to be a “native” pedant, but these days, I at minimum consider Flatpak a VERY necessary evil, if an evil at all.

NixOS or Fedora.