@Erpel As you say the improvements are marginal at best at this point. Tbh the thing that makes you replace a device is... software.
Like look at what Windows 11 did to perfectly good hardware. Same with Android and smartphones.
I am totally happy with my Fairphone 3+ right. But it only really has some community-support. So the choice would either be to run Android 15 until this thing is truely dead or get something new. But tbh when I bought it used. I just got a battery and its chugging along for some years.