Air accumulates in the reservoir, so you get a visible reminder to top it up as it gets lower
In my system thats as simple as unscrewing the top, squirting some distilled water in and rescrewing. done.
Thats what i just upgraded from on my 5800x, i ran PBO +200 (5850Mhz) and sat around 80C in stress tests, 85C in summer with the 3080
The 5800x is not a high wattage component, its just hard to cool - like running an older intel without delidding it, slapping bigger cooling just doesnt change temps much.
Moving to the custom loop i see around 60 in gaming and 75 in cinebench, with zero change if i start the 3090 mining into the same loop... it's all about that heat density and the core contact, not the size of the cooling.
The kits are pretty shit, best advice is to use them as a template for what you need to buy (number of fittings, etc)