I'm really sorry you had this bad an experience with GOG, and since I'm but an unemployed philologist publishing indie alt-lit books in a small publishing house, out of love, not getting paid at all, I'm absolutely not the person to give you tech advice and support, as you most definitely know how computers and the internet work better than I do. What I can say, though, is that you should give GOG another try. If you're that interested in Ultima VII, I think you should give it a go -- it's a very small file, and I doubt you'll have any checksum errors when downloading it, like you experienced with The Witcher. Adding to that, GOG's library of classics tend to work flawlessly under most modern OSes, overall, the Ultima series being part of those optimized good old games.
Not that it matters much, but I've never had any issues downloading from GOG, even with large installers... I generally use the GOG downloader and it never let me down thus far.
It strikes me as odd that a CS/Math major prefers the automated system of Steam over the control GOG allows over their files, but to each their own, I guess... personally, my slight paranoia tends to favour GOG's approach over Steam's; I like having to manually update my files, I feel "safer" having to install them myself and knowing where all my backup files are stored, in case anything goes wrong with the installation (especially when trying out mods). I installed the Steam client once, in order to redeem a code, and uninstalled it right away, as I didn't enjoy having that semi-DRM software installed on my machine, but that's just a matter of preference and taste, I guess (I'm aware Steam supposedly doesn't "spy" on us nor collects data we don't want it to collect, but, still, it felt strange to have a launcher for games connected to the internet, dependent on an online connection).