Check out Shelter. Iuse Shelter myself but I have heard that Insular does the same thing. Shelter has an option of installing from APK directly into work profile. I tried installing different versions just now and it worked.

Edit: Hadn’t properly read your post. Changed my reply to better fit the question

I think a large part of why Framework laptops worked and no mobile equivalent has come up is because most of phone ecosystem is not free. When I install Lineage OS on my phone I always expect a decrease in the quality of photos because lot of the camera stuff is proprietary and baked into the OEM ROM.

I get that the Framework equivalent of phones can manufacture their own devices and write FOSS firmware for it but it’s a big hurdle which not many can afford.

On the contrary in laptops/PCs you can install Linux and expect most of the hardware to work out of the box. So until phone ecosystem gets as free as PC ecosystem (I doubt it will considering how the corporations work) I doubt a Framework equivalent will be available for phones.

Quillpad is nice. I’ve been using it for about a year now. It satisfies all the 4 requirements as well.

Nextcloud by itself is pretty secure. They have a docker container to set everything up. So the weak link would be how you host it. If you host it on a vps, can the vps be trusted and if you self host it can you set the firewalls properly. Source: I’m currently setting it up on a Odroid N2+