I have the 12 and the mini and feel the same as the OP. The mini is very loveable and feels like there is nothing I can't do with it that I would do on a larger device. I often put both phones next to each other's and open the same app or website to compare. I constantly find myself thinking 'what is the benefit of the larger display now?'. Yes there is a little more content on a larger display, but you won't even notice if you don't compare side by side.
I still have not made a decision yet on whether to keep the 12 or the mini, but have a strong inclination to go for the mini at the moment.
Battery is ok, even the 12 will not last 2 full days, so I have to charge them every night anyway. And my mini shows 25 to 45 % battery after a day of regular use, before I put it on the charger.
Just to give you an idea of how good the mini feels to me: I had the SE2020 before (gave it away already). And when I compared it to the 12, I instantly felt that I wanted the larger screen. I almost never have that feeling with the mini. Oh yes, and the battery of the SE was much worse than the mini's.
What would be reasons for me to go for a bigger screen?
Extensive video watching & gaming
Extensive book reading (Kindle feels better on a larger screen with more content)
That's basically it. Everything else is perfectly fine on the mini.