Most planning mistakes for a Thailand wedding come down to treating it like a single-event Western wedding instead of a multi-day Indian one booking a resort without checking its track record with Indian ceremonies, underestimating guest logistics, or leaving vendor bookings too late for peak season.
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Assuming every beach resort can host an Indian wedding

A resort can be stunning and still be the wrong choice. Plenty of beachfront properties in Thailand are set up for a single ceremony and a reception dinner, with no real experience running a haldi, sangeet, mehendi, and baraat across three or four days. Couples sometimes find this out only after signing a contract, when it turns out the property has never built a stage for a sangeet or routed a baraat through its own grounds.

It's worth asking directly, before booking anything, whether a property has hosted Indian weddings before and how recently. This is also where researching an actual Destination Wedding Thailand planning resource pays off one that's worked specifically with Indian ceremonies will know which hotels have genuinely done this before, rather than which ones simply photograph well.

The properties that come up repeatedly for this reason JW Marriott Khao Lak Resort & Spa, Anantara Riverside Bangkok Resort, and Avani Khao Lak Resort among them have hosted enough of these weddings that their events teams don't need the ceremony sequence explained from scratch. That's a different level of readiness than a resort that's simply "done a few Indian weddings" in passing.