While it could at this point still be a battery fault, seeing as you’ve already replaced the bottom module to no avail it’s very much more likely an issue with the core module - which could indeed be considered terminal. They’re usually difficult to repair and FP doesn’t repair them as a policy, it only replaces them. And if you’re out of warranty, that’s a pricy fix.
Before anything else though, if you can, see if there’s any Fairphone Angels near you that can swap out parts to see if your core module suddenly springs back to life with different modules. The odds of that working are small, unlike the dinosaurs, but also unlike the dinosaurs, extant.
If nothing works, feel free to shoot me a DM as I may be able to help you get a new core module for cheap.