This is for your Audiologist to work out, it could be trial and error to find, there’s pressure venting as well you could ask about.
The Occluded Response with the Vented Dome M shows two things :
a) the probe tube moved while placing the hearing aids as you suddenly get a peak at 1k5 instead of 2k
b) it’s a very open setting for vented domes. They are too small, or the shape isn’t a good fit for your ears, another brand for the domes could be better
You seem to have pretty wide ear canals. I don’t see the point of getting C-shells, as SlimTips will do the same job and you won’t have to worry about sending the receivers for a repair since your audiologist can just change the receiver.
I do agree that 1mm is way too small for your hearing loss.
From what you’re saying, you might be a good candidate for Active Vent custom molds.
At least 3mm in real terms, preferably built as a collection vent or as an open side to the mould. Could go to a 4mm vent with vari-vent inserts if the AV isn’t practical. Then you could experiment with different levels of occlusion.
In the real world you’d probably do better with open(-ish) domes. Here, I’m finding your sort of loss works well with the smallest receiver sizes due to the lower floor noise filters that get kicked into the response: which mean your 500Hz hearing won’t hear the aid doing its thing.