I would suggest using a visa agent for your first visa. I did my wife's for the UK and got rejected the first two times. The visa applications are all about ticking boxes. My wifes failed twice on 'not having enough of a reason to return to thailand' so on third application we explained in detail that she needed to care for her elderly grandparent and it was accepted.
Just a quick update. I left the day after realizing the mistake with expiry date, by plane to siem reap. At the airport they transferred the expired visa and made a note of the one day overstay but didn't ask me to pay a fine.
I returned today back to DMK bangkok. Made sure i had all the required documents but didn't get asked for any and got stamped in with visa on arrival.
Obviously i wouldn't recommend anyone to re-try this method, and would encourage everyone to make sure you are looking at the right date of expiry 🙈
Thanks for the helpful advice and also the unhelpful advice 🙏🙏