I am having such a hard time in my mind. While I love the power, and storage of the new 2021 it’s a hard to justify a $1100 iPad Pro when a refurbished 2018 will do everything I need for $350 less.
For the 12.9" iPad Pro, I'm wary of the new display. So, were you dealing with that size, I would just go for the earlier iPad as it is likely to be more reliable on that front. With the 11" iPad Pro, there isn't that concern given that this is, more or less, an under-the-hood bump on the processor and the cellular hardware.
That said, 2018 vs. 2021 is all about how long you intend on keeping that iPad. It's likely that you'll get between 2-4 years of additional support for new iPadOS versions on the 2021 model, but you won't get to the point where the 2018 one isn't supported while the 2021 is supported for at least another 4-6 years at the rate things are currently going. Remember that the original 12.9" and 9.7" iPad Pros are both still supported. The former of those two is six years old and the latter is five years old. So, really, this is all about 4-6 years from now. If you only care to have this thing for 3-4 years, I'd say just go for the 2018 model. If you plan on using this iPad until you no longer can run a new iPadOS release, then go for the 2021 as that will mean you have this iPad Pro for a VERY long time.
Honestly if I want to keep the iPad for longer than 1 year I believe the 2021 would be the best bet. Twice the storage, Lidar camera, and will be more futureproof long-term especially as I want to keep it for 5 years or so.
Honestly, the 2018 iPad Pro will last you for well longer than a single year. It'll last you just fine for at least the next four years. The question is more, do you want to replace your iPad Pro in 4 years or replace it in 6-8 years? If you're going to replace it in 3 years anyway, you might as well go for the 2018 model. If you're going to use this thing until it stops getting software updates and you want to maximize that time, go with the 2021 model. Again, Apple is still supporting 5-6 year old iPad Pros.
or possibly just keep the refurb 2018 and by the time that limits me maybe there will be apps taking advantage of newer M series processors.
Again, how long do you want to keep this thing? The answer to that question dictates which iPad you should consider buying.
