From things rendering completely different from the preview, to straight up not rendering parts at all, random crashing during encoding, and so on, I am honestly surprised to see any positive feedback on this product at all.
Regularly use it (Resolve 17) on both Windows and Linux with larger projects than that (100s of 4K clips, multiple audio tracks, added effects, color grading and more) and haven't encountered the issues you've mentioned at all. When I try the beta versions (including the beta of 18), I've run into many problems, including seemingly random crashes, but never for the stable versions. Using AMD CPU and Nvidia GPU, multiple 4K monitors with a desktop PC.
> I am honestly surprised to see any positive feedback on this product at all
If I see software that many people are raving about but I can't even run properly, my first guess is always that it's my fault, or my hardware, or my Linux setup somehow. I think the same applies here. If most people can run it without hiccups, then probably something is a bit weird with your setup. If everyone had the same issues that you have, then they surely wouldn't have gotten the attention they are getting from everyone.
Have you tried reaching out to their support team or sharing your issues on the public forum?
I've tried it after every single clean re-install of windows 10 over the past few years, with every major stable and beta version they offered at the time, while never managing to not run into issues.
In the end I've come to accept that there simply must be some combination of effects and other things that I will habitually end up with that are not working properly together.
Gave up and just use ffmpeg directly for the few things I need.