Have you tried the S sleeves to see if they fit better than the XS? I have found going larger in size reduces feedback, but you also have to be comfortable as well. I found comfort level went down as you go up in size. Just ask for some S size to try. They are easy to change.
Feedback is a high pitch, not a rumble. What fitting formula are you using? I had a quick look in the software and the proprietory Smart Fit forumula at the Power level seems to be the one with the most gain. Here is what I get with it. The upper gain curve has about a 10 dB margin before it hits the feedback shaded zones (red and blue).
Here is what it looks like with closed sleeves. For sure you should not have feedback with them. You certainly could ask to try them as well. They have some advantages in noise reduction and microphone directionality. Just be sure that they program and turn on the Own Voice Processing as they can affect the sound of your own voice more.
As for molds, they are really not necessary for the gains you are using. Some find them more comfortable. I did find the canal only style fairly easy to put in, about the same as sleeves. I also tried some half shell style and found them quite difficult to put in. Much harder than the sleeves. In the end I did not like the molds and went back to closed sleeves.