As someone who does exactly this (I run LineageOS with microG) I can attest to the fact that a lot of apps just don't work without proprietary, non-AOSP compone

As someone who does exactly this (I run LineageOS with microG) I can attest to the fact that a lot of apps just don't work without proprietary, non-AOSP components being present on your device (GMS aka Play Services). microG helps, and does result in increased functionality of these apps (for now) however large parts of the applications remain unusable. AOSP is open source, "Android" the brand and platform controlled by Google is largely proprietary (if you want most of your applications to run properly).