On her computer, why not just use Thunderbird on it? Or even outlook, or whatever she likes. She just needs to pick the software.
On her phone, or even yours, why the stuff with accessing Thunderbird through vnc. Just add the server to whatever mail app on your phones?
If you want a web based thing, roundcube or sogo. But Thunderbird is gonna suck the way you are trying to use it.
I’ve made a few things. One, Mickey shaped cabinet hardware I put up for free because I didn’t want to deal with Disney. Though I might put the raw print up on Etsy or something just because.
Another is a frame to turn a Disney name tag into an ornament. I’m selling the completed product rather than releasing the design because I wanted to really offer it to people who didn’t have 3d printers and wanted to ensure that it met my standards. And I didn’t want competition. That said, it takes quite a lot of time to print, prep, sand, and paint. I spend a couple hours in finishing each and sell it at a price that… Totally doesn’t make it worth it.