I’ve used an iPad Mini 2 for about 9 years with almost daily use. Certainly, since it is no longer supported for new updates, there are hiccups, but it is still quite functional in many ways. Certain apps like Disney Plus no longer work, but other apps like HBO Max work perfectly fine. It may be slow at times, but for the most part, it’s reasonably smooth, with an occasional crash of the app here or there.
The version I have only has 16 GB of storage, so it lives on the edge of being full. This also means most apps are offloaded if not used recently and need reinstalling to use them. This is a minor issue if you mainly use it for watching HBO Max like I do.
Overall I am pretty impressed by the fact that it still works, but I wouldn’t recommend buying it. Disney recently considered the iPad Mini 2 obsolete, and who knows how long it will be until HBO does as well. This does means that if you get any current iPad model, it will likely be functional for many years.