Yes, technically the SE is more like a 6S, I just meant it looked similar to a 5.
Being out of date on iOS just means you miss out on whatever new toys are built into the later version, these free updates being one of iPhones' best value features imho. But as the saying goes.... what you've never had, you'll never miss. I still use a very ancient iPod touch, iOS 5 iirc, as a music player for the car. It still works perfectly and syncs with my libraries.
You might also miss out on security updates as and when vulnerabilities get discovered, up to you how much to worry about that. Apple have been known to retro fit super critical fixes to older versions, but generally they don't.
Of interest, did you manage the earlier battery replacement DIY, or use a repairer? I only once tried to dismantle an even older iPod touch, which ended badly - I wrecked the poor thing.