Honestly it depends on what you need an iPad for to answer the question whether it's worth the upgrade it not
if you are coming from an iPad older than iPad Pro 2018 and you need the performance as in gaming etc and you have extra cash go for it not only you'll get performance, but thinner bezels, new design better camera, lidar(if you need it) etc
Now if you have the iPad Pro 2018 or above or even the new Air
Then it depends
If you are iPad 12.9 user and you like to watch content on your ipad most of the time update to M1 will not only add Mini LED to your list but also performance and thunderbolt 4 along with center stage for video calls
If 12.9 is too big for you then other than performance and center stage you'll get slightly brighter screen with minor updates
A12x and A12z are still very capable even for gaming and will be for another year or so and with iPad OS 15 to my knowledge there aren't M1 exclusive features so you aren't missing out on anything, there aren't M1 exclusive apps as of yet although if you are into future proofing
My speculation is after A12X iPad Pro 2018, Apple's M1 devices are next best thing especially the M1 iPad
But as of now the iPad Pro 2018 is the best device in terms of future proofing given it's run and that's among all tablets out there it still is flawless
I own iPad Pro 11 2020 for me 12.9 is bigger the only thing that worries me a bit is my iPad losing its value as I keep on swapping my stuff but then again that's just me