To some extent I think you can do most of that on Lemmy.
You can upload images for your posts (not sure for short videos) and cross-post them to other communities.
Edit: I don’t know kbin.social but I suspect it would be more adequate to what you want to do.
Android runs on a unix kernel.
I personally think an open source android rom is a good middle ground between running a full linux OS and google’s Android.
I don’t know what roms are good right now though.
I would probably go for a fairphone and install the non google rom:
https://support.fairphone.com/hc/en-us/articles/9979154556817-Google-free-Android
Or check if your current phone is supported by any open source Rom.
In any case you will most likely have a lot less apps since you would have only access to non google play services apps. There is a fairly decent choice of apps on F-Droid but your experience will remain limited.