Can you put all what you said into one post please? So I can mark this as SOLVED for anyone wanting to know about this information as well?
IMO the Gigabyte Z390 Gaming X should be a great board, I'd rather have that than the ASUS Prime Z390-A that I'm using with an i9-9900KF.
I always buy very good (usually Seasonic) PSUs, but the
EVGA 750 GQ that you found is fine, but at that price level I would go with
EVGA SuperNOVA 750 G+, 80 Plus Gold 750W if you are in the US, good unit with 10 year warranty.
I'm not a liquid cooling fan so not the best one to ask. I use big cases so have a beQuiet dark rock pro 4.
That EVGA G3 that you found refurbished is made by Superflower, one of the best and I would be comfortable with a refurb with a 10 year warranty.
For a quick look at air or liquid cooling performance, I'd measure the available space, then take a look at the graph of various coolers air and liquid
HERE to see which will fit, cool around the green lined one -- the beQuiet, and will fit your budget. I wanted an air cooler that competes with mid range liquid but it is huge. It was also key for me that it fit over the top of 4 tall memory sticks, so look into that too. Anything above the green line result will cool quite adequately for an i9-9900 overclocked.
With the beQuiet Cooler the outer fan side sits just above your memory but the clearance is adequate -- you can place the memory first or after the Dark Rock but before the inner fan is snapped on. I just installed the sticks before putting the whole thing on.
Nice build.