There’s nothing compelling enough in iOS 15 to make the jump from 14 on an 8. Stay on iOS 14 until August- then upgrade just before the mess called iOS 16 drops.
In my opinion, the only time to install a newer version of iOS is in August when it’s done. Then you’ll only need to install it once when it’s “Finished” than suffer through the yearly time wasters of weekly updates all year fixing one bug after the next.
Launch day versions of iOS is for the birds- it’s never polished - it’s semi functional beta software.