So I finally decided to invest in some extra fans to do a positive pressure setup only thing is I'm unsure how to set custom fan curves in the bios.

I'm using a thermalright hub for every fan in my case, am I supposed to have the fans I want to customize plugged into the mobo instead of the hub?
And can you guys post some resources on some guides how to customize the fan curves properly because I can't find much online or at least specifically for positive pressure.

I wouldn't worry about custom curves too much.

Just run a static 500rpm, 700, 1000, then just do increments of 100 til you start hearing it/don't want to hear it and run it at the lowest (or highest) you can tolerate while playing a game or whatever you're running and keeping the cpu under 90 or 100c(ideally A LOT lower like 50-70c) and gpu should only be up when you're actively gaming.

Average gpu temp depending on the card even without system fans should be about 60-80c under load in a cool house of 70degrees Fahrenheit.

Other factors come into play like outside temp(if its freezing out and your PC is next to a window you can tag advantage if you're overclocking for points. But if you're just using it day to day and gaming don't sweat all this extra stuff.

Just have some kind of hardware monitoring software like iCue or hwinfo etc.

I agree to the above statement. But one thing you should make sure is to always have a good push and pull air flow setup. Bigger rtx cards and new cards needs more air flow and shouldn't be super tight in the case. Air flow from the front or side front in, top for air out back for air out and bottom for air out. Newer system generate a lot more heat and you want to be able to disperse it quick and still have a lot of air in, which is why you see a lot of builds with 3 air intake fans and big ones.

Always have something to watch your fan speeds, rpm and temps. You dont really need a custom setting as long as the fans increase under load.