Vegas 17 runs good and smooth, also with 4K HEVC files, on my laptop from signature with 16GB RAM available from which I never saw more than 8-9 GB in use while editing or rendering those files. Maybe time to reflect again and use the trial of the program to see if what you think is true?
The system requirements for Resolve or Premiere are pretty steep.... so to jump to those other editors only because you might think they will work better on a lower spec PC, I think you will be in for a not too happy editing experience.
Of course you can try the other software. Davinci for free. But Premiere you have to subscribe to and you will never own the software. Stop paying, and that's it.
That way you can see for yourself how they work on your computer.
I have all three, and Vegas is more elegant, in basic workflow, than both. The other two have obvious strengths.
What is your goal? What is Vegas lacking, right now, before upgrade. If you got the upgrade, with no change of PC hardware, what results would you hope for, improvement wise?
If you answer those, I can more easily give you a probably pretty good answer.