I have never had to replace thermal paste on any cooler I've ever owned. If you use a good paste, and do it right the first time, the paste will outlive the pc. About the only common usage paste that ever needs replacing is Arctic Silver 5, because after it's cured and hardened, even the slightest shift of the cooler will break the seal. That includes heavy vibrations from things like large fans or aio pumps.
Keeping a pc clean is important. Dirt and dust are insulators and once a blanket is applied to the cooler fins, it's going to lose efficiency, and temps rise. Especially important on blower type gpus, as they have fin channels the air runs down and small entrances. If the channel gets clogged, it's useless, no air flow.
4 fans is optimal. 2-3 intake, 1-2 exhaust. Depending on the case. The Fractal Design Meshify C is one of the best airflow cases there is. 3x 120mm intakes, 1x 120mm exhaust. The FD Define R5 is an awesome case, yet very mediocre airflow, 2x 140mm intake and 2x 140mm exhaust is good. So situation and fan size/placement is case dependent.
You can mount upto 10 pwm fans per header or 2-3 DC fans per header. So if your mobo only has 1 cpu_fan and 1 sys_fan, that's 20 possible fans total. You won't need a fan controller of any kind, that's a luxury. For 3pin DC fans, that's still 2-3 case fans and upto 10 pwm fans (pwn only on cpu_fan header), so again, no need for a controller, just a luxury.