I contacted FP support and the definite answer on this is as follows:
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Although there are JTAG pins/pads on the SDM632 chip, proprietary Qualcomm software is required to talk to the chip.
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The FP3 does not expose these JTAG pins on the motherboard, so utilizing JTAG to unbrick a phone is not possible (unless you were to unsolder the main CPU)
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Technical reference Sheets about the SDM632 fall under strict Qualcomm NDAs - not even Fairphone employees have access to them.
So - if your phone is bricked - JTAG is not an option