I recently switched from my 14 Pro to an Apple Refurbished 13 mini. In fact, today's the last day I can return it if I wanted to. It's nice going back down to a normal sized phone, even if I could only use it for a few more years before it becomes unsupported or whatever. I'm hoping to use this for at least two years, at least until battery replacements are no longer available, or a new mini releases.
As far as battery goes, at 100% capacity, I think it's fantastic. It lasts longer than my 2.5 year old 14 Pro at 82% capacity. Though, I noticed if the phone warms up due to the camera or any heavier activity, the battery will drain much faster than the 14 Pro would in the same situations.
As far as battery longevity goes, I can’t really say since I never owned a 13 mini before. However, I did own a 12 mini for two years. I think I started using a MagSafe battery pack with it around 6 months into owning it. I just slapped it on first thing in the morning, so it drained the battery pack first before the internal battery. I’m not a battery expert, can’t really say if this was good for the iPhone's battery or not, but it worked for me and always got my 12 mini through the day during heavier use.
These days, I use my phone a lot less, intentionally keeping it away from myself when I can, so I assume my 13 mini will last longer in these regards. My goal is to not use my MagSafe battery at all with the 13 mini. But, the option is always there if I need it. And might be a good idea for you as well if you think you'd need it.