It’s never been viable to upgrade the core module. Fairphone never made such a promise, and there’s plenty of threads on this forum where it’s explained how upgrading this piece is infeasible. TL;DR: unlike desktop processors with “standardised” pin-outs, you can’t just swap out an ARM SoC for another without redesigning the whole “motherboard” and completely redoing the Android build. The latter I’ve long been deeply unhappy about because it’s caused by SoC vendors cutting corners in their software design process, but either way the upgrade process is expensive, takes serious time, will become a support nightmare and goes against the idea of keeping the same phone for years.
Trying to build a phone that lasts 5 years is not the same goal as trying to build a case that lasts 5 years, but in which most users will swap out every component after 2. The latter is environmentally just about as bad as just releasing a new phone every 2 years.
FP has announced they’re working on a new phone to replace this 4-year-old design. It’s work-in-progress, and I’m sure they’ll announce more as soon as they have something tangible. I understand you personally are in a difficult situation. The life expectancy of the core module was longer than 3 years, but you got unlucky. And now you’re between a rock and a hard place. I’m sorry to hear this.
I think what you’d want to do is first ask yourself the question: was it worth it? Do you think that the 3 years you got out of your phone (with its ethically sourced materials, its wide range of operating system options… whatever matters to you when you pick a phone!) was worth while? Essentially: would you buy an FP3 when it’s on the market?
If the answer is yes, perhaps the most sustainable path forward is to find yourself a cheap second-hand phone to bridge the gap between now and the FP3 release. A second hand phone has already been produced and had a life, so from this moment on the environmental impact is smaller than anything else. And with a bit of searching you can probably find something cheaper than the core module with slightly higher specs.
If the answer is no, then I hope you will find yourself a different smartphone that is to your liking.