not a fan of the UI of the official project
I can sympathise here. GnuPG is an example for me, where third-party graphical user interfaces are used. But you are uncertain if they are secure.
For this scenario, you can use open source software. However, the unfortunate truth is you must review the code yourself and then use that code. Most developers will provide the source code on their website or via GitHub which you can compile yourself.
If I disable network access to the client could I be reasonably sure it won't phone home with my secrets?
Presuming the program's client does try to "phone home" it will have no Internet access. Hence, if you keep network access disabled permanently you can assure it will not be able to report your information back "home".
What you described above sounded like blocking that program from accessing your network, that's one viable alternative. Another could be using a virtual machine (less secure) or physical machine (most secure) to set up an air gap.