Why don‘t you just hold it in the proper orientation? It scrolls fine that way.
This issue is not a big deal. I remember reading about it when the mini 6 was new. I had one, and it never occurred to me to hold it like a phone.
I have several iPads now. Mini 5, Mini 6, Air 3, 11” and 13” Pros. They all do it to some degree when held in portrait mode. The Mini 6 is of course the worst.
But again, I don’t care because holding it that way makes no sense to me. That would be like connecting a 30” screen to my Mac and then turning it sideways.