What age is your V6, please, OP?
In the last 6 months I've gathered enough info to devote at least three chapters of my memoirs to coolant issues on my V6! Take a seat, get a cuppa....
Reasons for my overheating/high temp gauge readings being .... (in approximate order of occurrence)
1 Thermostat + transfer pipe - replaced
2 Coolant flush and associated - Air lock
3 Coolant temp sensor - replaced
4 Header tank cap - Slight leak
5 HBV (possible, changed anyway) -
6 Heater matrix (back flushed) - partially blocked
7 Pipe coming from rear of engine (under scuttle) - split leaking
8 The
other pipe coming from rear of engine (under scuttle) - split also leaking
9 Radiator - replaced with brand new (and cleaned old sensors mounted in rad)
10 Leaking main pipe - upon refitting replaced jubilee clip for wrong size (old one broke), following day replaced with correct size.
Each time anything was done to the coolant system that required a coolant dump, recover what I could, refill, and then often airlocks for a few days afterwards because I didn't bleed the system for long enough. You need to make sure the thermostat opens once, or even twice, ideally.
Eventually it was down to two main factors - the leaking pipes (which were a pig to get to, just a warning!) and a 21 yr old silted up rad. The others were probably adding
slightly to the problem.