I ended picking up a 2019 last year instead after further research the transmission issue was essentially resolved toward the end of the 2018 production run, but I wanted to play safe so got a 2019.  So far I've had it for a year and no issues.

This is more of my opinion, but I personally like to avoid the first few years of a new design incorporation on any vehicle, even on Toyotas.

I have not seen Toyota introduce a new generation/powertrain component without bringing new problems. A quick look at owner complaints shows me that transmission problems are the most common issues in the 2017 Sienna and really any car with that unit of 8 speed from that time. 2016 had a 6 speed auto why don't you look for one from that era?