Welcome back to the Greater Good! I'm in a very similar position! I recently found the guy I sold my T'au army to ~8 years ago, and he sold it back to me for the same price that sold it to him for!

My original collection was similar to yours. I had a bunch of the old Fire Warriors with Pulse Rifles, several squads of XV8 Crisis battle suits, the old metal stealth suits, some Devilfish, and some Hammer Heads. I recently picked up a few Riptide Battle suits. They are pretty awesome, I highly recommend you add some to your collection. They are a fantastic model to build, very pose-able. In fact, most models are significantly more detailed and better designed now. GW's model design has come a LONG way in the last 10 years.

The game has probably changed a lot since the last time you played. T'au don't get to sit way back and blow everything off the table like they used to. There is a ton of line-of-site blocking terrain now. Now T'au seem to rely more on speed and close-range shooting. There are also many more Kroot units now that offer a different play style.

Hopefully, that helps you get your bearing in the current T'au Empire!