I don't agree that the A12 to A15 is substantial (not even considering the fact that the A15 in the mini is downclocked compared to the iPhone and that A15 is not a big jump to begin with).
Nor is the RAM upgrade...
The move from mini 4 to mini 5 was huge in term of speed, like night and day, winter and summer. From A12 to A15 it's far from that gap, very very far, it's very much like the move from air 3 to air 4, not a big difference in speed, compared to moving from ipad air 2 to ipad pro 11, huge.
The move from 2 to 3 GB of RAM is pretty big and I can see it by comparing my 9.7 pro to my mini 5 (I don't even compare it to the mini 4 as A8 is slow as molasses...), with much less reloads while from 3 to 4 is not a huge difference. Again because probably 1GB or more is taken by the OS, so from 2 to 3 you are doubling the available RAM while from 3 to 4 you are not.
Also moving from no pencil to pencil 1 is a bigger move than from pencil 1 to pencil 2....Pencil 2 can be a big deal if you use it a lot, if used rarely, not worth it... you can find $15 pencil clones which use the Logitech crayon protocol.
The form factor is more aesthetics than utility. The screen surface is only 5% larger, the side bezels are actually bigger and the ratio is even wider than the 11 pro, so better for videos, worse for pdfs etc. Personally I hardly watch videos on the mini... The mini 6 is actually slightly smaller overall. If you value symmetry it's nice, otherwise not a big deal.
Headphone jack is a big deal for me, since I use it for my music libraries.
USB C is a big deal for me since I have a Samsung phone, but I have airpods pro for my Samsung phone, airpods 2 and another pair of airpods pro for my Apple devices... so it's not like I can completely do without lightning.... And I also have a lightning audio interface, which I precisely use with the mini.
If I had to buy one, of course I'd get the mini 6, even at the increased price for the same storage, even just for USB C, but upgrading from my mini 5? No need for several years (by that time there will be at least a mini 7).
But I have already a 11 pro and a 12.9 both 3rd gen... so definitely not a mini only user....