I use the iPhone for the obvious. Calls, email, texting, and short videos. Sometimes even games. My aging eyes appreciate larger screen size.
I first got the 10.5 Pro and that's my go to. It has cellular connectivity so I can take that with me anywhere. Videos, gaming, reading, note taking are my primary uses.
I opted to add the mini for a few reasons. Reading on the larger iPad is fine but I've used my mother's mini a few times and the Mini just feels so good in hand and it's perfect for travel in that regard. Videos are good but the 10.5 is my preferred option for movies. It uses the same Pencil that my 10.5 does so that's more use that I can get out of it, another plus. I really didn't need it because the 10.5 does everything I need it to and the Mini is wifi only model but I sure have been enjoying it.