Just to be clear:
The hardware interface will be a posix network interface.
It can & has been bolted on to the bottom of AX.25 & TCP/IP.
What ever protocol you want to use needs to talk to a network interface.
paclink-unix, used for Winlink support, uses a mail MTA, postfix, & movemail. There will be a minimal mail server (Dovecot) to allow support for pop3/imap clients. I have used our mail setup with mutt, thunderbird, neomail, claws-mail & K-9 mail on my Android tablet.
Not sure where the 'lack of focus' thing is coming from but let me just say you are wrong.
If you want to develop something and are familiar with BSD or posix sockets you will be fine.
We currently have the interface running on a browser using websockets talking to a node.js server in the cloud which talks to a daemon on the radio platform at my house. The components can be split apart & run on different machines or all together on the same udr platform.
There are many different ideas about how this box will be used. Since our group is heavily invested in this project you might figure out that it will do what we have the most fun doing. The software will be all open sourced so that you can make it do what you have the most fun doing.