Are the fans oriented right for good cooling though? Case airflow is critical: a GPU exhausting through the AIO radiator can impact CPU temp by 10 even up to 20 degrees.
For most cases the best radiator location is in front drawing cool outside air through it, then exhausting all the hot air at the top and rear. Some have a side mount and that works well too..
As far as thermal pastes go, it won't matter much more than 4 or 5 degrees even if you have the absolute worst and change to the absolute best...which tend to be conductive and either dangerous or ineffective if not correctly applied. Check out the comparison:
https://www.tomshardware.com/best-picks/best-thermal-paste
I have an ID Cooling 120mm AIO and I'm pretty happy with it. It keeps my PBO'd 3700X in upper 70's after 40 min's or so of video transcoding (handbrake). Gaming temps stay in the 40's or 50's and I completely did not expect that. I tested it with Cinebench for 8 hours or so and it got no warmer than low 80's which is still perfectly safe. IMO that's pretty good for a 120mm: a PBO'd 3700X puts out a lot of heat in a workload like that so I expected it to thermally saturate.
It does have a problem with mounting though: ID uses the old-style Asetek mount with their own twist. You have to get it evenly tightened or the waterblock can be slightly uneven with bad cooling a result. Make sure you're using the right plastic spacers and the four screws are evenly tightened all around. Also that the block is fully 'screwed' into the mounting plate. You'll know it's right if the logo is square to the motherboard.