It’s good to see that others are able to build the bridge utility without issues.
Some email clients provide an option for specifying the location or your mail director(ies|y) if Geary provides such an option then storing your Proton mail on a microSD card should be straightforward. Similarly, linking the mail directory on disk to a directory on microSD should be achievable without much issue I would think.
Some mail clients give you the option of initially downloading mail headers only and only downloading the full message when specifically view the message, this is helpful for conserving space but more of a hindrance if you need to do deep search within your mails.
Due to the limited capacity, it’s probably best to remove all the build tools and utilities that you installed for the build. As well as removing go which I mentioned previously, you could also purge the build-essential and git packages, also note that during the make some additional packages were downloaded, these were probably cached by go so you’ll probably have a directory at ~/go which can be removed.
I never went so deep as to check if the development libraries were statically or dynamically linked at compile time, and I don’t have access to the phone right now to check, but you might be able to remove those also.
Another potential space saving is that apt caches a copy of downloaded/installed packages, you might find some space can be reclaimed by clearing the caches found at /var/cache/apt/archives