If you're looking for a replacement for Soundbuds Flow (i.e. ~€20), you'll be disappointed. For me, Soundbuds Flow were perfect for workouts. Cheap and I didn't care too much about sound quality. Anker rebooted their audio line and focuses on mid-range to high-end products.
Is there any sense in installing a custom ROM on the phone to get rid of the software issues?
Custom ROM will help with some issues, but not all. If the issue is in a proprietary blob, like the random screen dimming issue that's plagued FP4 for months now, you'd still be stuck with the issue.
Kudos on Fairphone for offering 8 years of software updates.
As an owner of FP4, my biggest gripes with the FP4 are the software updates. Bugs keep languishing for months before they are acknowledged and then months pass before the bugs are fixed. Three annoying issues with my FP4 that I deal with on a daily basis:
I don't care much about Android 13 as long as I get timely security patches. What I want is a bug free experience and that's something FP4 fails to deliver.
An annoying hardware decision is the SIM card can't be hot swapped. Not sure why this wasn't addressed with FP5.