Big D Yes you can call or email to get a guarantee of benefits letter, I have an appointment for an annual checkup next month at Samitivej Sukumvit Hospital. Approval letter sent to you and the Hospital.
yes different rules for different immigration offices, i used the Ubon Ratchathani office, I even had a monthly deposit not marked international transfer and they said no problem.
With Bangkok banks app you can get yearly statement sent to your email next day, then take to your local Bangkok bank office and they will stamp it/verify it for immigration office.