I will be honest, I just gave up. I couldn't get consistent performance on HellDivers 2. Many of the things you have mentioned I've tried and found they don't make much of a difference or made things worse.

I did get it running nice for about a day and then an update was pushed and it ran like rubbish again. The game runs smoothly when initially running the map and then massive dip in frames for several seconds. This is usually when one of the bugs is jumping at you.

This game may work better on Fedora/Bazzite or <some other distro> but I find Debian to be super reliable and don't want to switch distro. I also don't like Fedora generally as I've found it unreliable in the past. I had a look at Bazzite and I honestly just wasn't interested. This is due to it having a bunch of technologies that I have no interest in using.

There are other issues that are tangential but related issues.

e.g.

I normally play on Super HellDive with other players in a Discord VC. Discord / Pipewire seems to reset my sound for no particular reason and my Plantronics Headset Mic (good headset, not some gamer nonsense) will be not found. This requires a restart of pipewire/wireplumber and Discord (in that order). This happens often enough I have a shell script alias called "fix_discord".

I have weird audio problems on HDMI (AMD card) thanks to a regression in the kernel (Kernel 6.1 with Debian worked fine).

I could mess about with this for ages and maybe get it working or just reboot into Windows which takes me all of a minute.

It is just easier to use Windows for Gaming. Then use Linux for work stuff.