domchu

1 - You should re-paste your CPU and GPU when you're thermal throttling pretty much all the time when the system is under load and it hasn't happened before, whether or not its non-conductive thermal paste or liquid metal. Only do this when you're 100 per cent certain its due to ageing thermal paste. From personal experience, I build my desktop 20 months ago with Thermal Grizzly Kryonaut and its still going strong. I've noticed that CPU temperatures were a few degrees higher than when I initially built it, but it could be due to room temperature and dust build up on the heat sink. I'm not planning on re-pasting my CPU. I did however re-paste my RTX 2060 with Kyronaut a week ago and temperatures have dropped 5 degrees Celsius in Time Spy Extreme, even though my GPU was running at perfectly fine temperatures, manufacturers often use lower-quality thermal paste than one from Arctic and Thermal Grizzly.
 
2 - I have never used liquid metal before so I cannot give a direct answer but a re-paste on a GPU with high-quality thermal paste like ones from Arctic and Thermal Grizzly will noticeably reduce temperatures, like i mentioned in my previous answer. You can also try undervolting which is quite easy if you watch a tutorial which will reduce temperatures and fan speed while maintaining the same clock speed or even higher clock speeds than before. My GPU temperatures dropped 7 degrees Celsius on Heaven Benchmark from undervolting.
 
3 - I recommend installing 'CPUID Hardware Info' and 'MSI Afterburner'. HW Info is just an overview of your entire system like GPU/CPU temperatures/load/clock speed and fan speed etc. MSI Afterburner allows you to undervolt your GPU like I mentioned in my previous answer and it allows you to toggle an in-game overlay of GPU/CPU temperatures/load/clock speed and fan speed etc. You'll have to search up the maximum operating temperatures for both your CPU and GPU models to determine whether your hardware is operating within those temperatures using HW Info and MSI Afterburner.

Sorry, it was the wrong screenshot, I uploaded the correct one now...

I recommend you avoid unplugging the PC. Modern PSUs are designed to always be plugged in, and there have been several people on this forum that have broken their PSUs by switching them off between every time they use the PC. The power draw should be about 0,05W-0,1W, so that's not really an issue. 

it's safe, and will consume little power while doing it

dont bother with the switch, just shut it down and leave it

I have little knowledge on monitors that are within your price budget but I recommend the MSI Optix MAG241C.
 
- 23.6 curved VA panel
- 144 Hz refresh rate
- 1ms response time
- 3000:1 contrast ratio
 
Its a great monitor, I'm using one right now but its out of your budget since its £179 in the UK so I assume it will be more expensive if you're in the US.

a DRP4 is a TON of cooler for a 8700 non-k.
You will be fine, even if you ran that thing at 100% all day long.