@David *****
Sorry but some of this advice is just incorrect or outdated.
Most immigration allow you to apply 30 days before a stamp expires. Some 45 days prior - a moot point for an exempt entry.
Some may ask you to leave it until the last 7 days - like Jomtien and Koh Samui but most won’t.
There’s no downside to applying early as you correctly stated they add on the 30 days to your existing stamp.
The 15 days via a land border using exempt finished in 2016.
Nowadays you get 30 days on entry using exempt whether by land sea or air - but are limited to two entries per calendar year by land or sea.
[edit only just spotted the “only two extensions in a calendar year”]. I guess that is a lost in translation thing. If you can get in using visa exempt you can get an extension (one per entry). There isn’t a limit.