I have some updates to report regarding adapters I tried:
Let’s start off with the best experience - this USB C (male) to 2.5mm AUX (male) cable by MOSWAG for use with my Bose QuietComfort over-ear headphones. The cable seems to work flawless when connected directly to the fairphone. The only downside I noticed is, that the 2.5mm side of the cable does not sit in the headphone plug as firmly as the original one - potentially due to the larger rim directly below the connector.
Now for the USB C splitters. I’ve so far tried two of those:
First off we have this two-way USB C splitter (1 charging, 1 headphones) by TRANSLIKS. When my FP4 has a significant amount of charge left the adapter works flawlessly with my default chargers for charging-only, headphones-only and both at the same time. However, if the battery has less than 25%-30% of charge left, a curious thing happens (depending on the type of charger used). For my old Motorola (Nexus 6) and Apple (iPod Nano 4th gen) chargers, the connection seems to disconnect and re-connect in short intervals once the charger is connected, which also interferes with the headphone functionality. With my Laptop’s charging port and a more recent OnePlus charger on the other hand it worked fine all the way down into single-digit battery percentages.
Secondly I tried this three-way USB C splitter (1 charging, 2 data) by MOSWAG. While listening to audio output and charging at the same time did work with this one when I tried it, once one of the two was disconnected the other one stopped working as well. Also charging, listening to music and transferring data to my laptop in parallel is not a task this adapter seems to be able to fulfill with my FP4.
Finally I tested a USB C to USB C (charging) + 3.5mm AUX (audio) adapter by the FP4. I chose this adapter/splitter by Tomost since I quite like the ‘parallel’ design of it, almost giving it the feel of an internal audio-jack. Unfortunately the audio jack seems to be incompatible with my FP4 - tested again with my Bose Quiet Comfort headphones.
So much for that. I hope some of you find this follow-up information useful - especially the USB C splitter stuff, since nothing had been reported regarding compatibility with those so far.