180 wouldnt be bad with the correct fan settings.
cracks at 180, fully open by 195. Fans on low at 197 and on high at 205.
Otherwise go for a stock temp.
Ethylene glycol and dex don't get along.
Flush it well if thats whats in there.
Dex isn't bad as long as its maintained.
I just pull off the lower rad hose. My drain cock has been stripped.