Best to have your side fan with a dust filter on it, so you dont clog up the gpus
ideally id add a fan controller, give it access to ambient temps and inside case temps, so the fans can run using delta T, it helps confirm if you are getting enough airflow into through the case, if you are then as above look at dropping the voltages if able on the gpus while you max them
the fans on the rad are not pretty, id again check the temp, of the rad and the temp of the air coming out of the rad, if its hot wack on stronger fans, if its not then perhaps look at changing the thermal paste on your cpu, it might not be transferring very well
that rad should be strong enough to cool the cpu with the right fans, im not surprised its improved with that front fan changed
as stated elsewhere adding better fans on the side is great, but check all your fans cfm so you can try and get close to the same intake as exhaust, i normally try for slightly higher intake, helps with dust issues and does not stress the exhaust fans
