The SE is a fantastic phone now, it will carry on being a fantastic phone for some time to come, iOS 12 won’t be a problem and I doubt 13 will either. Personally I feel like I’m over the need to constantly wait on rumours and worry about lifespans, we’re well past the point where things develop that quickly anymore. It’s not like the SE2 will have a 128bit processor and 10gb ram, it’ll be the same setup as the iPhone 8 minus a few features, which really isn’t miles away from what the SE is now.. The reality is, whichever one you buy, you’ll probably sell on and upgrade years before it’s even remotely stretched.
OP, open the box, sell your 6s while it’s still in sellable condition and hasn’t died on you altogether. Relax and enjoy your new, more pocketable phone.