I have Corsair 280mm (2 x 140 fans) installed in the Corsair case. There are different models (with RGB or with Elite Capellix LEDs etc.). My friend has H150i from Corsair and a friend has one from NZXT (z73, but costs 280 euros). The new one from Lian Li is also good. If airflow is good, a cheap Alphacool or one from MSI is also sufficient, the CPU doesn't get too hot.

Yes, that is completely sufficient. I have the Kraken X63 water cooling (2x14cm) - CPU temp +/- 40-60deg.

Well they are literally the same CPU (only difference is the Turbo Clock Speed; i9 10850k: 5.2GHz / i9 10900k: 5.3GHz), the i9 10850k's are just those which didnt hit 5.3GHz Boost under ideal conditions in quality tests

Many factors can play into this. For the first time, the RTX 3080 is a lot better than the 6800 (about 15-20%). In addition, it is quite possible that you still had some background programs open, these also play into the overall score. Try closing most of the programs in the background and do the 3D mark run again. Also take a look at the thermals, i.e. the temperature of the CPU and the GPU, there may be thermal throttling here.

No