Transmission service question

Just wanted to see if anyone has prior experience with this.

I inherited my 2014 mirage with 200k KM on it from someone, who had purchased it from a precious owner. It runs okay, it needs a few fixes but overall it's fine so far.

My question though is that I am not confident that the transmission was ever serviced, from either owner. I have no record of it and they don't know.

I've heard that if a car is older and hasn't had the fluids changed, it's better not to touch it because trying to put new fluid won't work out well. Anyone have experience with this?

And while I'm at it, any tips on other things to do maintenance on that might have been missed over time? Beyond the usual oil and filter change.