do the timing belt service if it hasnt been done and change the atf

For better gas mileage, check the tire pressure, clean the MAF sensor and change the air filter but basically your driving habit is the most important parameter for the gas mileage. You may also consider changing the spark plugs and coolant as well. 

I had a 2000 3.2TL from new, it was the most reliable car I've ever owned and I've owned a Tacoma, and Accord, a Corolla etc

Unfortunately I totaled the 3.2  TL at 115K miles (that car saved my life).  I never had a single repair the entire time except  regular maintenence of course.

You bought the last model year of that (3rd) generation (2004-2008), smart choice as usually bugs/issues are typically ironed out by then.  Too bad the base TL dropped the manual transmission as some of the automatic transmissions in that generation had issues (although it was really pronounced in 2003 2nd Gen), so hopefully you had the vehicle (especially transmission) thoroughly checked out before you bought it.  Be sure to stay on top of the transmission fluid changes (drain/fill/filter change) and take it easy on the transmission (don’t drive it like a maniac), and it may last longer.  

https://www.motorbiscuit.com/acura-tl-the-most-common-complaints-you-should-know-about/

https://www.cashcarsbuyer.com/acura-tl-problems/

https://www.carcomplaints.com/Acura/TL/

https://www.carcomplaints.com/Acura/TL/2008/

https://www.carcomplaints.com/Acura/TL/2008/recalls/

https://www.carcomplaints.com/Acura/TL/2008/tsbs/

https://m.youtube.com/watch?v=4ki4eQ4a3Os

For gas mileage, have you always noticed mediocre/poor (not sure that’s the right word to use since I don’t know your mpg) gas mileage or is this something new?  See below for some things to look at:

https://m.youtube.com/c/ScottyKilmermechanic/search?query=Gas%20mileage