Use a decent, high-performance coolant. There are threads posted of what people have used.
Flushing just means filling and dumping out the coolant until it’s clean. I’m not sure about vinegar and water.
It is a sealed system so there really shouldn’t be any gunk inside, but you never know after 15 years. Crazy things grow everywhere. You might discover a new type of anaerobic, flesh eating bacteria and release it to become the scourge of mankind on this planet.
But not likely.
Do some googling and searching on refinancing G5 systems. There are some good YouTubes out there.
I’m not expert. Results not guaranteed.