immigration doesn't like the DTV program. They were not consulted before it was announced and implemented. They don't want anything to do with it, including processing extensions for it. Most offices will tell you to just do a border bounce if you want more time. For those that persist and submit an application, they can expect a long list of requirements that changes each time they return to the office, until they finally give up and leave.
If you already have a Bangkok Bank account then your plan should be okay.
Expect to be confronted by your bank and forced to sign a document that gives them permission to freeze your 800,000 baht for 4 months per their new policy. Anyone trying to get proof of funds for immigration on money that has not already been in their account for 4 months must sign this document to freeze the money for 4 months from the day the letter is issued.
Suggest showing them what you think of their policy by trying to open an account at a different bank and using the new bank for your 800,000.
You can get a re-entry permit but it's only going to be valid until the end of your 90 days. You need to be back before that ends so you can apply for the 1-year extension.
Your entry stamp will be for 180 days. One time each entry you are allowed to apply for a 180 day extension, but pretend that option doesn't exist and just expect to leave every 180 days and return. Many immigration offices do not want you to extend and will make it all but impossible.
it's an entry requirement for Thailand which means if the airline flies someone to Thailand who is denied entry for failing to meet those requirements, the airline can end up being forced to fly that person out at their own cost.
The same reason airlines check for visas, travel authorizations, vaccine requirements etc.
You will receive a 60 day visa exempt stamp if you arrive without a visa. This is eligible for a single 30-day extension.
One requirement to enter as a tourist is proof of onward travel, which is a ticket out of Thailand within the number of days you will be stamped in for (60).
Immigration almost never asks for this, only if you have a history of trying to stay in Thailand for a long time without a long-term visa.
Airlines much more often ask for this proof, and many will not let you check-in for your flight without showing proof of a ticket out of Thailand. Will they accept a bus ticket? That's up to the airline. Every airline sets their own policy. Immigration would accept it.
Have you received a warning before? You're around 5 months in the last year, so it's a little iffy. But generally you will receive a warning before being outright denied entry.
I would recommend having the 3 proofs if you want to enter.
1) Proof of funds (20,000 baht or equivalent in another major currency) IN CASH ONLY
2) Proof of onward travel (a ticket out of Thailand within 60 days of arrival)
3) Proof of lodging (at least the first 1-2 weeks of a hotel booking)