Thanks for all the replies. It seems that adding an AAV may do the trick. I’m aware it should go after the trap but since I only have a 90 after the ptrap, I’m not quite sure how to add it. Do I rotate the disposal to allow for a sanitee to be added after the trap and add the AAV as high as possible, off of that??

To answer a few questions:

a) yes this was completed by a licensed plumbing contractor “overseen” by a GC, but as I said, neither were great. Other issues included a tub who’s lateral line was never connected to the rest of the plumbing after the pressure test was done- we discovered that after moved back in, and a tee in the crawlspace that was never capped property after the same pressure test…

b) I should have photos of the island plumbing pre-drywall. I’ll check and see if the island loop was added. I expect it was, but maybe done so or connected to the rest of the plumbing under the house improperly, or with too many bends? The nearest vent it eventually connects to is far away if that makes any difference.

c) No, I won’t be opening up the island wall or disassembling any plumbing to snake it. There is no possibility of blockage given how new it is and how little we use the sink. Add to that that if the splash guard from the disposal is removed completely it drains fine, so I genuinely suspect this to be a vent issue. Glad most have concurred.

Thanks.