Strongly seconded.
Apart from the very negative Fairphone 1 Android upgrade experience caused mainly by Mediatek, people perhaps interested in this aspect should ask themselves why Fairphone (kind of) competitor SHIFT promises support to Custom ROM communities for their 6mq phone, but not for their 5me/ 6m phones … spot the difference 
.
By the way, having their 5me/ 6m shipped with Android 8.0, thanks to Mediatek they are stuck at 8.1 according to their own support.
And a LineageOS 15.1 try for the 5me/ 6m went into trouble shortly after its start and looks abandoned for a while now (see here).
(I personally don’t care much about the Android version number as long as everything I need works and I get security updates, so nothing wrong with 8.1 in my book until Google stops supporting it.)
In Qualcomm land …
Fairphone shipped the Fairphone 2 with Android 5, did an upgrade to 6 … (with that, Qualcomm stopped support for the SoC) … managed to do a Google certified upgrade to 7 anyway, apparently manage to do a Google certified upgrade to 9 (release candidate stage currently).
While official (in their terms) LineageOS for the Fairphone 2 went from Android 7 to 8 to 9 to 10 currently, preparing the release of 11.
The Fairphone 3 got /e/, a LineageOS based Custom ROM, reasonably quickly after the phone’s release, with the e foundation behind it and Fairphone even setting up an official partnership. Official LineageOS followed.
/e/ and LineageOS were made compatible to the Fairphone 3+ quickly, too.
You are using a Custom ROM yourself.
It’s in your own interest Fairphone stay with feasible SoC choices regarding that.
Open hardware would be nice someday in the future, though.