What are the advantages of applying for the Long-Term Resident (LTR) visa before entering Thailand?

You can post questions on the BOI site and they respond quite quickly . My LTR visa was approved within 2 months of my application

As Tod mentioned, best to get the stamp in your passport once in Thailand at the One Stop service Centre in Bangkok . Easy to access by the MRT system

There is no "pre-screening" you start the application process on the LTR website,

you upload the required documentation, and they will contact you if you need to provide more documents or supporting documentation. You don't pay anything until you get approved

Now that is NOT the case using the eVisa website thru the thai consulate in your country 😮 You PAY up front and whether approved or rejected you lose the application fee

If you apply for an LTR in your country BEFORE you come using the eVisa online website you don't "fly anywhere" it's all online thru the web-portal that all thai consulates use to issue visas now 🙁

My advice is DO NOT USE the eVisa online web-portal thru a thai consulate in your country for the LTR, that's because you pay a premium fee to get the visa thru that consulate

AND

all you get is a PDF that is emailed to you <- that's your VISA for the next 10 years that you carry with you every time you exit/re-enter the country. There is no "hard stamp" of a visa/re-entry permit if you buy the LTR thru a thai consulate

Instead I would say use the LTR/BOI website they have dedicated to that visa.

You apply thru the LTR system direct online (which you can do inside or outside the country), wait the processing time and once it's approved go to the LTR/BOI office in Bangkok located at the new One Bangkok building to get the 10 year LTR visa inked IN to your passport along with a 5 year entry stamp and a 5 year re-entry permit

Here's the LTR website,