If it was me, I’d contact Fairphone support and explain to the agent your situation, and hope a compromise can be reached (ie. a discounted repair). You are only just outside the warranty period after all. While you shouldn’t get your hopes up, better rule out the easier and potentially amicable option before trying to prove there is a manufacturing defect.
We all know there is a problem with at least some of the FP3 and FP3(+) core units, but extracting blood out of a stone would be easier than getting a venture capitalist company to admit that I’d have thought ???. I’m not hot on consumer rights law, but I do know that here in the UK the onus is on the claimant to prove that a manufacturing defect exists. I’m guessing you bought your FP+ from Fairphone not a third party seller?