Try doing a battery replacement. I bought one off Amazon for $20 and it came with all the necessary tools. The process was pretty easy if you watch a few guides on YouTube beforehand.
I finally bit the bullet and upgraded from a SE, compromising on the size of the phone. I already notice increased wrist stress and place more sideways weight than is optimal on my pinky (to hold the phone up but also be able to reach single-handed).
So, knowing how life works, we can guarantee that Apple will release a small form factor phone as part of their next offering ;)
best. smartphone. ever.
(I actually just dug mine out to give it another go, but the battery is nuked from sitting untouched for eighteen months :\ )