Old iphone SE ios Update and Battery?

My main phone is an iphone 13 mini.  Have an older iphone se first generation as a backup phone and also have a sim card on it.  So have two phones i use with two separate plans.  The one with the older iphone se, i pay very little for that plan.  My main phone is my mini.

Now the battery on the iphone se is very bad.  The battery health is 83 percent.  Had a battery change from apple years ago... this was a bit less than 5 years ago maybe.  So battery has been bad for many years since then.  Now i had this phone for over 6 years at least.  But haven't used it as my main phone in quite some time as i been using the mini.  Now i am still on the old 14 ios and never updated to ios 15 years ago because people say it will make it unstable.  But some comments say it isn't true?  The thing is this old iphone is still usable... use it to youtube at home and use it more than my iphone mini because i just prefer the smaller size of the old se.

My question is should i update the ios or not?  What about a reset of my iphone or should i not do that?  Really don't want to do that but would it make it much faster?  Again this is the phone i use to youtube at home mainly on wifi.  The mini is the one i use outside etc.  Now the battery doesn't bother me because it isn't my main phone.  But what would be the cost to replace the battery and is it even worth it?  Again this old se is like a backup phone but it does have a sim card where i pay monthly for limited minutes.   Now the current ios it shows is 14.8.1.  Now it also shows i can upgrade to ios 15.7.5.  Thoughts? 

Hello, @paulyjustin:

Before anyone here goes down the rabbit hole trying to answer your multiple questions, do you already have a post elsewhere, e.g. at the Apple Community, perhaps under a different username?

If so, please provide a link to that thread.

Respectfully submitted

Have not yet at the moment.

Early bugs in a new iOS release are common but get fixed in point updates. iOS 15 was very stable (before I updated to iOS 16). In addition, Apple is still fixing some security bugs in iOS 15 for devices that are incompatible with iOS 16. I suggest you to update for that reason alone.

 

Now the battery on the iphone se is very bad.  The battery health is 83 percent.  Had a battery change from apple years ago... this was a bit less than 5 years ago maybe.  So battery has been bad for many years since then.  Now i had this phone for over 6 years at least.

Is the battery health currently 83%?