How can I demonstrate payment for a 3-month rental when applying for a Non-O visa conversion in Thailand?

The process works like this;

You will apply first for the 90 day Non-O visa for 2000baht at the Immigration office.

You will get a date stamped on the blue/green receipt paper for the 2000baht telling you to come back for the "results" on a date 2 weeks from the day you apply (that's why you need a minimum of 15 days on your current stamp)

You go back on that date, they ink in the Non-O and the new 90 day stamp.

Then you wait until you have 45 days or less left on the 90 day stamp and your 800K baht is seasoned in the thai bank account in your name only for 2 months.

You apply for the year extension for 1900baht and they issue it that day right then..

After that you get a re-entry permit if you want to exit/re-enter during the year extension validity

Just so we're clear; EU isn't a country 🙁 it's a geographic area) and not all countries in the EU are treated the same as far as thai visa stuff 😕

Also you only need a minimum of 15 days left on your current visa exempt entry stamp or 30 day extension from that stamp to apply for the in country Non-O

Be that as it may,

IF you have a TM30 filed, have the lease (minimum of 6 months) have the land lord i/d, house book listing of that property, and all the rest of the documents, copies, updated bank book, bank letter, filled out TM87 form you should be good to apply for the initial 90 day Non-O visa for 2000baht

I don't think you'll have much of an issue with a new year lease, and proof you paid rent along with the documents from the landlord