A bit late in the year, but you might find something interesting here: https://www.operasiam.com/
Looks like there is a New Year Concert in December.

Thailand Philharmonic Orchestra has a number of concerts in December: https://www.thailandphil.com/events/

From the looks of it 'Bangkok Festivals' have a very full line up ... but unfortunately not in December 2019!
Might be a good idea to keep this link just in case you come in a number of months that are NOT in December. http://www.bangkokfestivals.com/en/press-release-2019/