Due to thermals? More due to the fact these consoles use seven year old CPUs. People won't "try" 4K. The game will have 4K support or not. You don't "try" stuff on consoles. You don't have that kind of option, unless developers include something like a performance/movie mode with frame-rate or resolution preference, but this is unlikely in a racing game. My guess is that ACC will only support 1080p on consoles, but we can only speculate at this point.
Your HDR comment is really weird. First, HDR should not impact performance. This is not how HDR works. Secondly, HDR looks pretty good in this title. Maybe you don't have configured it right. Last but not least, running ACC at 120 frames must be quite an epic task on a MSI 2070. I own a 2070 myself and i can achieve 60 fps in 1440p on high details, which is fine. 120 fps sounds a bit far stretched. Are you running the game in 720p?