The only possibility for an issue is at initial check when you're departing but even that is unlikely since you have a return flight booked. They should be aware of the 30 day extension option or multiple other options available (exit/reenter Thailand to other countries as part of your trip). Almost zero chance of a problem but in the
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,000 chance, the onward ticket sites offer temp tickets for $20-$25. I'd bet no problems tho.