How can I extend my Non-O visa based on marriage if available online slots are full?

Yes.

Generally if you have everything prepared meticulously you have your one year extension done in one day, taking a few hours from arriving at the immigration office and waiting.

However it is quite often so that there might be some requirements some paper required some procedure to get the paper that makes you have to go and get it somewhere else and come back another day.

After some years you've learned every intricacy of every rule and every requirement and you can more or less nail it!

But expect to learn and possibly come back several times so don't leave it till the last moment.

For example if you need to season the money make sure that you have the right amount of time etc. make sure that the money was transferred in the required way typically wire transfer etc if that's the requirement of that immigration office...

This is a general list, it is NOT office specific and you would be well served to slog on out to C/W before you're going to apply and get the hand out from Section L-1 out there.

That way you know exactly what they're wanting

The online booking system in Bangkok books up FAST, because people finally found out how much quicker it is to make an appt for their extension

AND

The system only shows appts one month out, This morning at 12:01AM the next day (a month out) opened for appts <- which is why you only see Dec 26th as available for marriage extensions next month. All the rest were booked up...

You can keep checking the system because people cancel appts and they come back up in the booking system as available.

BUT

other than that you can just show up, queue up with the zombie horde and wait it out for your extension 🙂

Here's the bangkok specific appt booking system