After a quick YouTube search for battery drain tests, I found that the 13 Mini generally lasts 6.5 hours with nonstop use. The S21 generally lasts 7 hours with nonstop use. Your S21 has better battery life. You'd better off just getting a 13, or 14, both of which will last much longer than the S21, basically twice as long.

It might be a little more money, but the phone itself will last for several years without slowing down, so you won't need to upgrade as fast as you would with a Samsung. Samsungs are great, but their lifespan is much shorter than iPhone.