Can I apply for a 30-day extension on my 60-day tourist visa as soon as I arrive in Thailand?

'When' you can apply for extension depends on the immigration office you intend to use. In the past I have done it within two weeks before the stamp expires.. Once I was turned away from the Pattaya Immigration Office and told I had to wait until the day it expired. I used an agent in that case. I think the pattaya immigration office is more crooked than other immigration offices. Also I have a pretty questionable visa history so that may have been a contributing factor.

I did mine 2 days into arrival. No problem.

Pertaining to your TR visa, is your current return flight within 60 days or already 90? I’m curious if the airline & immigration will accept 90 days or will require 60 day return which will cost you to change the return flight after the extension.

Raises a question with me..owing to the fact that this year my multi entry visa (after having 7 in years prior and subsequent to..the new e-visa system..(I came anyway) and received extension, and happily close to borders in the North where a border run is relatively easy to do..My question then, in future years I may apply for 60 days instead of a multi entry ..do they have a shelf life as the multi entry does..in other words if awarded a 60 day visa..does one have to use it right away on arrival..or wouldit be better to start with a visa exempt..then initiate the 60 day (actual) Visa after the 30 day exempt award??

Mostly you have to wait until you are down to 30 days or less, a couple of Immigration offices allow 45 days and some just 7 so it depends where your TM30 is registered

You didn't say which immigration office you'll be applying through. Most offices allow you to apply when you have 30 days or less left on your entry stamp. Some allow you to apply when you have 45 days or less left. A few only allow you apply when you're on the final week of your stay (Koh Samui, Jomtien).

None of them will allow you to apply as soon as you arrive since you'll still have 60 days left.