Can I extend my Tourist TR Single visa in Thailand even though it expires before my travel dates?

You must arrive at Thai immigration any time up to Feb 18. The clock starts ticking on your 60 days from your date of arrival. So if you arrive eg Jan 7, that brings you to ca. Mar 7. Before that date seek your 30 day extension at the immigration office to bring you to eg April 7. If you still need more time then the “married” route maybe best and the effectively the same physical process of going to the immigration office (but with some more documents).

Only other way that comes to mind would be to exit the country and re enter on a visa exempt stamp if you are from a participating country of the exempt list. But Covid rules can make this last one risky. Best to get the extension without leaving the country. Good luck.

The O/P is married to a thai so they could qualify for the 60 day visit thai family extension. They can come here Jan 8th, get stamped in on their single entry tourist visa until Mar 8th, take the 60 day visit thai family extension for 1900baht and that will get them almost to May 8th. which is in line with their travel plans

Though looking at your travel dates that will still not be long enough.

The visa is valid for 3 months from the date of issue, when you arrive you will be stamped in for 60 days and you can then extend for a further 30 days.