Recommend checking out the SailfishOS[0] by Jolla. I have daily driven it on Xperia XA2 and XA10 II and it works "well enough" and is mostly open [1]. I have bought the OS so the phone has Android app support. This review resembles my experiences well [2].
E: I noticed that currently purchasing SailfishOS from outside ofl Europe is "prohibited" according to the purchase page. That is a bummer for non-europeans.