Well, you are going through a logical thinking process in this regard. I think it is good, in this situation to be somewhat skeptical and you are clear in expressing what you are not so sure of. I've done some reading on this. But of course, it's not a subject I'm highly familiar with either. What he's telling you is that you have a 'FAT' tongue and he can remove part of it.
His advice is to have HIS surgery FIRST or INSTEAD of chin advancement. Mine is to have the chin advancement FIRST and his surgery later IF the chin surgery does not assist in better breathing. Now, ASSUMING that chin advancement would yield an aesthetic improvement whether or not it also improved breathing in your case, my decision analysis goes something like this:
Chin surgery first. If you have the chin surgery first, there are 2 basic outcomes:
1: Aesthetic improvement with NO improvement to breathing.
2: Aesthetic improvement WITH improvement to breathing.
Either way, you get aesthetic improvement from it. IF #1, you confirm his diagnosis is right and you then STILL have option of having his surgery. IF #2, you confirm, you don't need his surgery.
Tongue surgery first.
1: Improvement to breathing but no aesthetic benefit.
2: No or not much improvement to breathing and no aesthetic benefit.
Either way, there is no aesthetic benefit to this surgery. IF #1, where it did improve breathing, you have to ask yourself if you would later want the chin surgery for more aesthetic benefit. IF #2, no breathing benefit, you would then later need to explore the chin surgery for better breathing.
So it DEPENDS on what is WORSE to you to find out FIRST and whether or not you would want the chin surgery just for aesthetic improvement.
Would it be worse to you to find out first the tongue surgery did not improve your breathing and just resolved to having to get the chin surgery later because it didn't work OR would it be worse to you to get the chin surgery first and find out that although it improved your aesthetic, it did not improve your breathing.
To me, personally, I would go for the chin surgery FIRST, which has an aesthetic benefit, on the CHANCE that it could also improve breathing. It would be WORSE to ME to have the tongue surgery first and find out THAT did not improve breathing than to find first that the chin surgery did not improve breathing and especially so IF I would get the chin surgery ANYWAY for aesthetic improvement.
Keep in mind that it's a decision under UNCERTAINTY and the choice resolves to asking yourself what you would rather NOT find out FIRST.
Here are some links I found about tongue surgery and also a device to try which might help determine if the tongue is the culprit.
https://www.fauquierent.net/botcoblation.htmhttps://www.verywellhealth.com/tongue-stabilizing-device-for-treating-sleep-apnea-3015243