Can I enter Thailand as a tourist and then apply for a marriage extension of stay?

You could enter on the 90 day visa waiver stamp that Chilean passport holders get BUT you couldn't apply for the year extension from it. In order to get a year marriage extension you first have to hold a Non-O visa

It's a multi-step process in country and goes like this 🙂

1 - you come in visa waiver,

2 - you apply for the 90 day non-O visa for 2000baht

++you wait for the under review period to run down

3 - you go back and get the Non-O and new 90 day stamp inked in

**you wait until you have 30 days or less left on that stamp

4 - you apply for a year extension for 1900baht & get a 30 day under consideration stamp

**you wait for that time to run down

5 - you go back and get the year extension inked in

OR

You BUY a 90 day single entry Non-O visa based on marriage to a thai from the thai consulate. (you could go to a nearby consulate like Savannakhet Lao and get a 90 day non-O no issues for 2000baht and minimal documentation)

THEN you could come in get stamped in for 90 days on the Non-O and apply for the year marriage extension when it runs down for 1900baht. Getting the Non-O visa before you come here cuts several steps out of the process I outlined above