Can I apply for a 30-day visa extension in Bangkok now or should I wait until closer to my expiration date?

Bangkok allows you to apply for a tourism extension when you have 45days or fewer days on your current stamp. You can go to apply any day now.

I've been turned away for visa extension if more than 14 days before expiry date. But each immigration office is different.

Doesn't matter in my opinion. All immigration offices require no less then 30 days remaining on your existing stamp before they issue you a 30 day extension of stay based on tourism.

>can apply for the extension now in Bangkok by going to CW,

You would use Muang Thong Thani Immigration office IF you are using Bangkok.

%C2%B055'07.7%22N+100%C2%B032'56.7%22E/@13.6,100.43,17z/data=!3m1!4b1!4m5!3m4!1s0x0:0x0!8m2!3d13.6!4d100.3

Easiest way is BTS to Wat Phra Sri Mahathat station, then get a cab from the roundabout under the station.

>o I need to wait until close to the expiry in mid December and apply at my nearest immigration office (likely Phuket or Samui)?

You can do it there as well. All 30 day extension of stays based on tourism are issued on the spot. You will need the following:

1. Your passport and:

a. a copy of your data ("picture") page

b. a copy of your most recent visa (if you have one)

c. a copy of your most recent stamp

d. a copy of your departure card

2. A TM30, filed in the immigration office.

3. The forms (each immigration office may have different forms, and want to use their special forms)

4. 1x passport pictures (some immigration offices require 2x passport pictures)

5. 1,900 THB.