I wonder what DIY options are like?
As the owner of an ageing 5S, I've looked into this myself. As far as I can tell, the main difficulty in replacing the battery yourself is getting the old one out, since it is glued in quite firmly. I've read numerous stories of people setting fire to their phones by accidentally piercing the old battery while trying to lever it out. This is especially a problem with old batteries that apparently get quite soft.
There's also the problem of lots of fiddly wires to disconnect and tiny screws to remove, then put them all back together. There's one particular screw that is shorter than the rest and apparently if you put the wrong (ie too long) screw back in that hole, it can cause damage to some delicate components.
Basically, it's not a job you should undertake lightly. I've replaced screens on various phones over the years but replacing the battery in a 5S is a job that makes me nervous. There are numerous YouTube guides out there if you want to give it a go though.