Are the safari split screen pages on the smaller ipads still mobile sites in iPadOS? Other than that, yes it’s the same software experience (if that’s what you meant), but the overall experience sure isn’t identical. Screen size is always going to be a major factor in a user’s experience. Whether the screen size difference between the 12.9” and 10.5” is considered “significant” or not is subjective, but to put a number on it the 12.9” screen has a 51% bigger surface area.
Yeah it’s always a compromise between productivity and mobility. One device can do it all for some but not everyone. I also need the bigger 12.9” size for what I do, but I also like having a very mobile casual device, so I have a mini in addition. The combo works out pretty ideally for me, but everyone is different.