Despite no Custom ROM support there’s an effort to port LineageOS … if you’re curious how that may play out, you can follow proceedings here … SHIFT5me - SHIFT6m - LineageOS 15.1 (Android 8.1) | SHIFTPHONES
Regarding The Mediatek part … Choosing a Mediatek SoC was the prime reason Fairphone had to stop software support for the Fairphone 1 early.