Hello, I read your description several times.
Ok, by points, so
1. The program may be running from the flash card only; not sure now but if the flash has own hardware code - I may try to bind exact version of the program to be work with this one flash stick only; so even someone powerful will copy it - it will not work on another place.
2. I think to make standalone binary exe / dmg using crossplatform qt5 or lazarus
3. Attractive ui - I think to use some html5 template so in future you will hire nice designer who will change their view
4. Will be two programs, one - the flash compiler that will place a copy to selected flash; second - the program itself that will run from flash drive in macos or windows as you have described.
Ok, the budget may be negotiable in both cases as well as time to complete, though I avoid rush work.
Thanks, have happy day / Eugene