Unless there are very specific reasons to stick with the FP3+ and its specifications (e.g. the audio jack which the FP4 doesn’t have and the FP5 certainly won’t have either), the necessity of a core module replacement would most likely make me change to a new Fairphone. The extended warranty on the FP4 and most likely also the FP5 is probably increasing the chance to avoid such a costly “heart transplant” repair bill like the one needed on your FP3+.
On the question what to do with the defect FP3+, you could consider offering the parts still fine and working in the forum’s unofficial marketplace. Every once in a while, people are even looking for parts not available on Fairphone’s official spare parts shop like the rear module (I only just sold one, I hope it’s arrived by now or arrives soon). The camera+ module is also a module other Fairphoners might be interested in, both the top module+ and the speaker module (little known: the FP3+ speaker module is a bit louder than the original FP3 speaker module, although I doubt if that has an effect when combined with an original FP3) and last but not least the bottom module. The battery is better just traded locally or nationally because of legal restrictions to shipping a battery.
And if you don’t care about making a bit money by selling the modules, you could donate them to the nearest interested Fairphone Angel.