I think closest you can get to a custom is the the Ultimate Ears Fits.
Those look interesting but not quite what I'm looking for. It sounds like those offer a better / more-customized fit, but not the full custom fit experience. To be honest, I'm not sure how much that incremental difference would matter, but there's a degree to which I want to get the complete custom-molded experience if I'm going to go down this route. Definitely intriguing and I appreciate the suggestion.
I haven't seen any. Only one diy pair a while ago.
Why not get customs and a Fiio UTWS3 or TRN BT20S Pro?
I didn't know those types of products existed; thank you for the recommendation! I'll have to look into them more. Have you used anything like those or seen any particularly informative reviews? I haven't used any earbuds with over the ear loops like those before so I'm a bit hesitant about that aspect, but it would open up a lot of options in terms of the earbuds themselves.