It is unlikely that "all the work that would need to be done" to make the PinePhone as useful as an Android phone (even with pure libre software) will even get done. The problem is that the PinePhone is just too underpowered in CPU and RAM, comparable to devices from many years ago. Plus, the PinePhone dev community just doesn’t appear to be large and motivated enough to cover all the bases of e.g. battery optimization that the corporate mobile developers have done.
>no powerful browser beyond the clunky Desktop-Firefox-for-Postmarket-OS hack, etc.
I don't really know what you mean by this. It's the exact same Firefox that's in desktop Linux. You can install all the add-ons and such. Do note that Manjaro is not PostmarketOS.
The biggest problems are the weak CPU as you've mentioned, and the fact that the entire OS is in a very alpha (or even pre-alpha) state right now.