oota is underdark, which is a landmark of FR at least fir me it is and has always been the feature which makes it different from other vanilla settings. No matter if ghk also had its underdark and prior to FR.OotA's Underdark is the Swordcoasts Underdark. So that leaves just Tomb of Anniliation for Chult. So that is really just Swordcoast (including Underdark and Chult). And FR has more fans then all the other settings combined, by a large margin, 38% vs 5% with 55% Home Brewers cherry picking elements of FR and other setting to add to their own. Get a proper FR setting book out of the way, THEN move on.
underdark in FR is much more prominent. To have that covered is super important, and Oota does that quite well. Might be that the oota area is underneath sword coast, still nothing more easy than moving it somewhere else below the earth.