Yeah I Hear you there... £500 is enough to be spending, especially around this time of year!
My main concern is, if the coolant temp/car think the temp isn't optimal does it then postpone doing dfp regens? leading to possible issues with that? (I'm just guessing I'm no mechanic!)
I'm not sure where the Thermostat is on these, or how much coolant is lost when changing etc.., so not sure if its a job where you'd save a bit of labour by having it done at the same time as the Cambelt?
With my other cars in the past, I've left it for a while as they never had dpf's thankfully.. this one, however does.
I'm burying my head in the sand with mine as the Mrs uses it for work, I barely go in it & it does get to 90 albeit slower than i'd like, but I think it drops a bit when stationary at the moment & it's mpg reading still seems fine.
I might be worrying about nothing, my a4 gets to 90 quite a bit quicker, but then the older engines may run hotter?.. who knows
Oh & in regards to the driving of the car, the A4 & golf that had properly fluctuating temp gauges both drove absolutely fine & also never noticed much in the way of mpg drop (hence i left them for a fair while.. rightly or wrongly) but both were fixed & stuck to 90 after putting a new genuine thermostat in.
Shame the newer a3 doesn't use the same one as i have a spare one here for my a4.. thinking back, the temp gauge in the A4 did have a funny turn a couple of years after the first replacement, so i bought a second thermostat (was only £25 at the time) to have put in when i had the cambelt done.. but i forgot to buy the new seal. So the mechanic didn't swap it at the time & actually, it's been ok since (touch wood)
So maybe.. a cambelt change/coolant change & a bit of disruption might actually kick yours back into action (if it is in fact an issue) never know lol