Lenovo IdeaPad L340-15API, no-SSD version.
Win10 Home X64.
Completely fresh install of everything. Galaxy version of the game.

Texture quality resets to High upon hitting 'Apply'. Each time.
And I doubt that altering config manually would do anything on Galaxy version since Galaxy overwrites everything (including re-downloading saves I deleted) prior to game launcg.

It's of little help, but texture quality is the least of your concerns. I think what it does it take up space on your video Random Access Memory, and doesn't have that much of an impact on the performance itself. Unless your device is very low end, I suggest keeping texture quality at High/Ultra in video games.

This sounds like more than just an issue with Cyberpunk, but possibly with Galaxy if it's overwriting settings. However, my guess is that it's not actually overwriting settings; what you seem to be encountering with the save thing is the cloud sync, which detects whether all of the saves are on your PC or not and redownloads them accordingly.

Have you tried actually manually altering the settings or not? If you have, you'd probably do best to contact GOG support but if you haven't, it wouldn't hurt to give it a try... and check to see whether the settings file is read-only or not. Actually, this is incorrect. If your GPU has a lot of VRAM, then you can run with higher textures. VRAM is different from standard RAM, it's faster than typical RAM and is on board your GPU. As such, it's used only for visual effects and the number one graphics option that fills up your VRAM is texture quality. In a very oversimplified explanation (I'm not an expert), your storage drive loads the texture files of whatever area you're going into (thus why load screens will pretty much always be a thing in games) and places it onto your VRAM so that the GPU itself can render the textures onto your screen. With VRAM, it'd have to load into RAM or directly via your drive on demand, meaning you'd have major slowdown.

So no, you need to know how much VRAM your GPU has and adjust your textures accordingly. I believe 6GB of VRAM (EG: a GTX 1060) is recommended to play the game with textures on high.

Any updates on this? Did you manage to fix it? Because i am having the same issue

To be honest, I'm on amazing quest for Job of my Life since November - so I didn't had enough spare time to wrestle games-broken-beyond-redemption.
Given exceptionally alarming changes in GOG's policies and overall behaviour, it's extremely unlikely that I'll ever return to this problem, this project and this publisher.
There was a rumor that there's a working graphics settings fix (and, overall, hardware-friendly tuning) at NexusMods. I'd suggest looking there - and hoarding antidepressants in advance, because applying these may fumble in spectacular fashion.