No need to wait. The current ones are phenomenal. It'll be at least five years before any of the apps that you'd use will start having issues in any way.
After four years with no new versions, there probably won't ever be one.
Even the original 2015 Shield is still one of the top 5 Android TV devices ever made, with the 2017 and 2019 models also taking spots.
I did add the newer remote to my 2015 model as that's a big improvement (other than the annoyingly easy to accidentally press Netflix button, but I remapped that to Show Recent Apps instead).
I really love mine a lot. I bought the shield 2 or what it's called and it was the same with a better remote. But after using it for a few month, i couldn't get it to connect to the internet anymore. Not wireless and not via ethernet cable. No idea why. I'm using the oldest shield and in my humble opinion it's quite flawless.