Maybe there is some issues with some charger and brand : I have read that some Samsung device are heating a lot when quick charging, maybe the battery is damaged in this case.
For FP3 maybe the quick charging devices and process have been carefully checked by Fairphone …
Concerning the Eco mode do you know if it is a hardware or software “tool” ? If it’s a software one then changing the OS will remove it I guess.
I believe that with time smartphones are “loosing” the real capacity of the battery which is why it has to be re-calicrated from time to time.
This XDA link explain how to do with Li-ion battery but does not say why it is required : https://forum.xda-developers.com/showthread.php?t=2192948
Note that it is an old article, but I think it is steel applicable.
I have bought the Fairphone Cable and I will plug it to my computer as much as possible to load my FP3 or with a non quick charge device.
Thank you for sharing your point of view.