Thank you all for your feedback! Just to make it clear, the title of my post could easily be rephrased as "2018 vs 2021 iPad Pro: is it worth upgrading?". I think the "Convince me" language riled some people up, but at least it got a fruitful discussion going with good arguments on either side.
My final decision, based on some of the input here, was to keep the 2021 iPad Pro and trade-in my 2018 model at my local Apple Store. I was able to get nearly $500 for it (after 3 years!), so the math worked out for me to make this upgrade and future-proof my setup for several years into the future.
If the trade-in value wasn't as high, I may've likely kept the 2018. The used iPad market's pretty hot right now, and that's what made this pencil out.
The Apple Store provided me cash value (rather than a gift card), since I used my Apple Card / Apple Cash, which also helped make the decision to trade-in. Lastly, being able to get the device physically inspected in front of me at an Apple Store, rather than shipping it out to some untrustworthy trade-in dealer, put me at ease as well. I've had bad luck with that in the past and this time it was a much smoother, more trustful experience.
Coda: if Apple has such a generous return policy (if you choose to call it that), that's on them. They've already priced their products with the return policy in mind, and they have an efficient device refurbishment/recycling system that turns around open-box items easily and cheaply. It's all of our right to purchase a device, try it, and return it if we want. There are countless posts here with buying advice that reiterates that.