Can I return to Thailand on a visa exemption after obtaining a SETV if I have already booked an onward ticket?

-Dutch passport

-First SETV which is stamped till dec 6 (ie without the 30 day extension)

-Previous visits all short yearly holidays on visa exemptions, last one a year ago

To obtain our SETV we booked a ticket to Luang Prabang after 58 days as proof of onward travel. We would like to stay up to 90 days and then move on, but we were not sure whether booking it around 90 days would be OK.

If we cannot reschedule the ticket and actually want to use it, is it likely that we can come back in on a visa exemption for the final 3-4 weeks? As this is our first SETV I think it should be no problem but I just want to check. We have no intention staying longer than 90 days and will probably only come back at the end of our trip (in June).

Thanks!