I went from the 2018 to 2021 as well and I agree with your sentiments. Everyone said that 50g wasn't noticeable, but when I put the new iPad in my first gen MK, I felt the added heft.
I don't know if this is necessarily true or not, but it does seem more top heavy on the MK, and it tips a little easier than with the 2018 on my lap. Now, it could be all in the head, but that's what I first noticed as well.
In terms of display, I'm sure if I ONLY watched HDR content then it would be 100% worth it, but in everyday usage, I really don't see the difference... Oh, I'm sure the difference is much better than the 2018 model, but in the day to day common things I see on the screen, you aren't blown away by it, let alone notice that it's a mini-LED screen
Finally, the battery seems to drain faster (obviously due to the screen). That's a big downer for me because Zoom already drained my 2018 pretty quickly. I haven't had the opportunity to use Zoom on battery power alone, but a 2-2.5 hr Zoom call would drain my 2018 to about 50% battery. I will have to see if the brand new 2021 fairs similarly.
On a positive note, this thing FLIES. Yes, I had the 2018, but even that had small hiccups here and there. This one shows no slowdown when opening and using apps and/or having many tabs on Safari. Also, having a bigger storage is also worthwhile.
At the end of the day, if I had paid for the iPad out of my own pocket, I would have returned this 2021 for a 2020 model on sale. As an iPad enthusiast who LOVED my previous 2 iPad Pros (2017, 2018), this one has the least value and emotional contentment for me.