Phuket is very crowded and offers little fishing opportunities for DIY'ers. Seasonally, there is excellent C&R sail fishing and good other inshore fishing esp. if you get away from the main resorts. (In the old days, you'd take an overnight boat to the Similans which was a fishing and diving paradise, but I believe that's a national park now and no fishing is allowed. At least legally . . . .)
I don't know Krabbe, but in recent decades it's become nearly as built up as Phuket. If you are really keen (and seeing how it's your 50th), look into booking a liveaboard charter out of Krabbe north into the Andaman Sea along the Burma border. I've had non-fishing friends make this cruise for 3-5 days and they report pristine reefs only the odd dynamite fisherman. That option may or may not be on the table, but December should be the start of calmer weather (less wind/rain) so perhaps there are charters to be had.
Inland, Ian Forbes (March Brown) fishes for mahseer on the fly inland. Thailand is heavily populated, so if you want to fish in decent solitude you'll need to be off the beaten track . . . Ian seems to ride a motorbike and is probably well Upcountry.
Around Bangkok, there are a handful of pay ponds that have all kinds of exotics stocked including South America arupaima and giant Mekong catfish. There are videos on YouTube showing fishing tourists with saltwater spinning outfits landing 200lb beasts . . . and letting them go (of course).
The Gulf of Siam is pretty much fished out. I'd skip it.