I haven't already flushed the system because I haven't had the time necessary, will do it right before replacing the thermostat, my actual one is stuck on open so that would be an advantage when flushing the system, and for the last refill, I will be doing that once the new thermostat would be already installed, want to flush it more times to have it as clean as possible, also flushing the radiator separately. Also, I am not sure I have a hgf because as far as I know, if that's the case, coolant would be leaking into the oil most of the times and not the other way around, I am suspecting an oil cooler issue or inlet manifold gaskets...
Regarding the flushing method, I'll be using this guide:
KV6 flushing
It's pretty straightforward and easy to do, but instead of draining through the oil cooler pipes, I'll be using the lower radiator hose (and cylinder block drain plug of course)