I am sobered by your “If someone put you in open domes I would . . . be worried. Unless your ear canals are very small. Like, pediatric small.”
Well, but you’re not in open domes. You’re in tulip domes which may be appropriate depending on the shape of your ear.
However, you comment that if you push them way in you ‘don’t hear ambient noise’ which sounds a bit like a lack of gain. But then you also mention that you are turning them way down. So it may be the case that the low frequency gain is appropriate, but you are removing it?
Hearing aids can take some time to get used to, and more time if you’ve been putting them off for a long while. Just communicate what you are experiencing with your clinician and keep in mind that adaptation takes time. They may be able to adjust things a bit, reduce some high frequency gain or add some compression in a way that makes the hearing aids more comfortable, and then slowly transition you up to more audibility.
As for the original question about whether you need custom tips–you’re on a borderline where they may be beneficial or you may hate them and the clinician is making their best guess with probably a minor bias towards not asking you to spend more money. I’m sure that they would be willing to try them with you, you’d just have to be willing to spend the money and not get angry with them in the event that it doesn’t work out (custom tips are often non-refundable). The potential benefits if indeed the tulip domes aren’t supporting you are a richer, clearer sound with better access to soft speech. The potential downsides are being unable to hear anyone when you are chewing because everything is way to crunchy, you own voice is uncomfortably loud and your footsteps are boomy, a slightly less discrete device. You could try a power dome first to try to get a better idea which way it’s going to go. Power domes are often more occluding than a properly vented custom tip, so if they cause you problems then a custom tip still may not, but if they DON’T cause you problems then you’d be fine in a custom tip.