since you wanna use vms and you got time to tinker, take a look into proxmox
adding a ssd shouldn't help transfer speeds afaik.
to the best of my understanding how truenas/hexos works is
transfers go to your ram first then to your hdd, bottle necked by the hard drive speed.
if you add a slog ssd it would go ram -> to both slog drive and hdd but unfortunately it is still bottle necked by the hard drive.
slog drives only copies whats still on the ram. This is mostly for in case of power loss nothing on the ram is lost.