Everytime sound dubbles, it adds 3 decibels. So lets say you have all fans locked at 30db. That means you already have 5 fans at 30 db only in the GPU area. Now with the given information instead of 30 dB you'd get around 36 db as sound output. So you can lower your RPM as much as you want, but its more efficient to have 3 fans of your GPU spin a little faster and get at 36 dB. This way your GPU gets wayy more air, so your cooling solution is more potent that way. So with that given, the ASUS TUF is still the best option to fit into the NCase because of the bigger heatsink and just better cooler design. And if you are looking to get even more cooling potential out of your card I would suggest undervolting it. Optimum tech did a video about this very subject, and he got a 2 degree drop in undervolting, so it may not be the best solution for better temps, but you can get a little more cooling out of your card this way.
Hope these links will help a little:
Optimum tech's video about a TUF card in a Ncase
Optimum tech's video about undervolting
Or look a little back on this page and you'll find some deshroud builds, you can also look a bit into that option, but it isnt worth it IMO, for the ventus the temps are just not ideal so you would be better of with the stock TUF, and deshrouding the TUF will involve bending some tabs and maybe damaging and (i think) voiding waranty doing it that way, with the same temps but little more quiet, so maybe its worth it but thats in your hands