I have a 12.9” and mini, and the combination is great. But as you alluded, it matters what your other devices are too. It's the entire device ecosystem that one has to consider.
This is my device usage breakdown:
- SE 4”. Used everywhere except at home. Serves all my miscellaneous needs should they arise while I’m out and about daily, in the most non obtrusive form factor possible.
- iPad mini. Used only at home—unless I know I’m going to do a significant amount of casual reading when out of the home, which is very rare. This basically replaces my phone at home, because I like a small phone when I’m out, but a huge phone when I’m home. Handles all my miscellaneous needs, but especially messaging, e-reading, casual browsing, and playing unimportant or non-cinematic video. At home, the mini is by far the device I use the most (unless I’m working from home). (Note- this sounds minor, but it’s important to me: I need to use the mini with a pop socket. It turns out I don’t really like using it without one. This is because I like handling the mini one-handed, which I find difficult on its own.)
- iPad Pro 12.9”. Used at home, at work (as a secondary device), and when traveling. It is my main personal productivity device (usually with keyboard), and handles some light work including mobile drawing. It is used for entertainment (casual reading, video) when traveling only, so that I don’t have to also bring my usual preferred entertainment devices (iPad mini and TV).
- MBP 15”. Used at home, and if needed when traveling. Heavy work, file and media hub, digital library management, and time machine backups.
- TV. Used at home, obviously. This is exclusively where I watch any long-form video that I care about.
The mini won’t fit into everyone’s device ecosystem well. This is the particular device setup that serves me the best, and the mini has a star role in terms of enjoyment and percentage of time used. So based on your needs, consider which devices in your ecosystem are the mainstays, and build off those. Since you have a larger phone and a smaller iPad Pro than I do (if those are mainstays), the mini’s value is less cut and dry for you than for me. Only you would know based on your usage, but it’s definitely still possible that there is room for a mini in there.