Because it's just for personal archival, I would recommend recording in super high bitrate 264 or 265 depending on what your card can do in real time, then compressing that file later using either av1 or 265, depending on which works better for the content.

If you're playing in 1440p165, then you should record that to start although if it's an option I would play and record at 1080p120. 1440 is a bit of an odd duck resolution that some stuff doesn't really like, but it's getting better all the time so it's not a huge deal. More important is 120fps because if you decide to go down to 60 or even 30fps they split evenly, which is important since it means the extra frames just get thrown out. There's no 82.5 frame so it either has to blend two together or pick one that's at the wrong time, and neither looks good.

Record in OBS using like, 25-40mbps in whatever your graphics card can do realtime, with all the audio tracks in something lossless like FLAC. High enough it might as well be uncompressed.

Once you have your gameplay recording from OBS, use either Handbrake or FFMPEG to convert it to your long term storage format. Since this is such a big project I would make some samples using cheats to get an idea what the bullet-hell-iest parts will look like, then try a few different handbrake or ffmpeg settings, and see what gives you the tradeoff of file size and quality that you like. It'll also give you a ballpark idea about how much long term storage you'll need.

Non-realtime encoding like ffmpeg and Handbrake is much more efficient than realtime done by your graphics card, like on the order of like half the file size for the same quality - that's why you want the two step process. It also allows you to play in 120fps for that responsiveness, but watch at 60 or even 30fps to save some file size.

When you set up handbrake, you'll have a few settings to play with and make samples from - encoder, quality level, speed, fps, and rescaling.

Encoder is probably the most important. Use either x265 or SVT-AV1. In my experience, they're close in terms of efficiency, but AV1 pulls ahead in certain situations. AV1 is more efficient about large static sections and when it breaks up it just looks blurry. 265 is better at retaining texture, but when it fails its gross digital blocks. 265 is faster than AV1, but when you push AV1 and it takes 1000 years it does a better job. Basically, run a lot of tests and then decide.

Scaling is where you go from 1440 to 1080 and is probably the least important for file size. I honestly wouldn't bother with it, but you can try. Like I said, technically some TVs don't like 1440, and everything supports 1080, but I wouldn't worry about it too much.

FPS I have the least experience with. My guess is that going do something lower is going to save you some size, but its really going to depend on the codec and the content.

Quality level is the most important setting - just set it to what you think is watchable. It doesn't use a bitrate, it adjusts the bitrate to get a constant quality level, so it's much more efficient. It's important to note that they aren't exactly the same at every speed setting, so be careful.

Lastly is the speed. 265 goes from very fast to very slow, and AV1 goes from 9 (fast) to 0 (stupid slow). Personally I use slow or very slow for 265 and about 4 for AV1, but if you have a lot of video to crunch through you might want to crank that up a bit.

For audio use OPUS. 96 is the default "bitrate" and I find it to be enough. Some players don't like it but if you're thinking about using AV1, anything that would support AV1 supports opus.

The main thing will just be doing a bunch of video encode samples and finding the settings and workflow that's right for you.