This reminds me that I never replied to your original thread. Firstly, custom is definitely the best option for maximum sound isolation so you're on the right track.
There's two ways you can go with custom. You can get either custom tips that you can use with your IEM of choice, or you can get full blown CIEMS. Obviously either route will require getting an impression done at an audiologist, but that's not too expensive. If you decide to go the CIEM option just keep in mind that they have basically zero resale potential. Sorry but I can't recommend you any specific pairs of CIEMs because I've never had an interest in picking any up. The main downside of tips is that the IEM sits a bit further out from your ear (if you care what others think).
Also if it were me I'd blow my entire budget on the study/whatever else IEMs because they're the pair you'll be able to appreciate the output with. I'd even be keeping an eye out for second hand IEMs. You can get some really nice audio gear well beyond your budget cap if you're happy to shop used, and audiophiles tend to be way more trustful than someone selling their used (not used for mining!!!) vega 64 w/o receipt.
I skate/run with some trash $4 philips buds that were all the rage on budgetaudiophile like 5 years ago and they honestly aren't that bad. I actually prefer them over my se425s for any outside fitness stuff because I'm more aware of my surroundings as the rubber tips don't isolate any where near as well and things like wind noise, cars, etc are loud enough to hide a majority of the difference in sound quality anyway.
Anyway, gl.The research grind for this type of shit can be an overwhelming time sink but at least you know it'll be years before you ever have to do it again LMAO