I have a proxbox with a R5 4600G even under extreme loads the stock cooler is fine. Honestly once prox is setup you don't need a GPU. The video output of proxmox is just a terminal (Debian) so as long as things are running normally you can do everything through the web interface even without the gpu. I do highly recommend a second GPU (either a G series CPU or a cheap GPU) if you want to try proxmox GPU passthrough. I've done it and can say it is extremely difficult to get working reliably with just a single GPU.