To the OP, the custom molds on the OPN does have a small vent to help reduce occlusion. There is also closed dome option with one or two small vent to try out if you don’t want to go with custom molds during your trial period yet on your OPN S1. But to fit you with open dome/fit on the OPN based on your hearing loss is not really the best approach. No wonder you get feedback issue with the OPN S. Neither the custom molds with a small vent, nor open domes with a small vent, gives me feedback on my OPN 1. And I don’t feel much occluded at all as long as there’s a small vent.
You mentioned that you don’t have feedback at all with an open fit on the Phonak Audeo M90. That’s pretty good given your very severe high frequency hearing loss. I don’t know how Phonak does it, but I wonder if that’s why you don’t like its sound quality of the Phonak right off the bat. For example, you said that you cannot hear the parking sensor in your car with the Phonak, but with the OPN, this sound is annoyingly loud. So perhaps the feedback control on the Phonak, while effective, comes at a cost of the sound quality somehow.
I think frequency lowering will help you a lot. The frequency lowering of the Phonak (Sound Recover 2) uses frequency compression, while the frequency lowering of the OPN (Speech Rescue) uses frequency tranposition and composition.
Without going into too much details of the differences between these 2 types of frequency lowering here, I think the Sound Recover 2 from may be better for your particular hearing loss because you already have significant hearing loss at 1KHz and even worse at 2KHz. Sound Recover 2 can extend the lowered destination pretty low (even lower than the 1KHz area), so it’d be more in the audible range of your remaining hearing. Speech Rescue from the OPN can only go as low as 1.5 KHz, not as low as Sound Recover 2, but not too bad. I’d say if you’re still trialing the OPN S1, try out Speech Rescue anyway. There’s nothing to lose. If it doesn’t seem to help, you can always go back to the Phonak and try out its Sound Recover 2 to see if you like it better or not.
I still think the key to helping you improve your speech understanding will be to try out the frequency lowering feature. It’s designed specifically for people with the heavy and early ski slope hearing loss like yours by moving the high frequency sounds into the more lower audible hearing range that you still have.