@Dowly
da sagst du was! 
Serious -l started to consider upgrading the gfX board alone, with a 4060TI card. I "chatted" with ChatGPT and gave it my precise system data: CPU, mainboard, PSU, and so forth. It says that a 4060TI should fit well (plugs, power, spacing), and should also be harmonic with my CPU. Well, that info may be artificial, but whether it is intelligent remains to be shown... I will see.
i7 8700K 6x3.7 GHz, Mainboard Asus ROG Strix Z370-F, 32 GB RAM 3000, gfx-Karte Asus Strix GTX-1080TI OC, SSD Samsung 960 Evo 250 GB, HD WD Black 2TB, PSU BeQuiet! Dark Power P11 750W, CPU fan Noctua NHD15 .
I am just not sure if my problems are realyl due to the gfx board alone, or something beyond that, in which case I would have spend the money - ~400€ - in vein.
Anyhow, I just did a SMART test of the system/Windows 10 SSD, it never was loaded with more than 50% of of the volume, has a status of "good" (94%), and 9200 running hours since autumn 2017, which indicates all that it is fine and healthy and stable and most likely has many more years of life ahead of it. The mainboard is a very solid one and said to be of good and lasting quality. So is the PSU unit. Its amazing how far SSDs have come.
I think I take the risk and try it with buying the GFX card. After all, I am lucky enough that I can afford it. And I am absolutely not in the mood to buy a new Windows 11 system. Absolutely not.