If possible get a certificate of residency from Thai Immigration.
If you're going to use 400K or 800K deposits to qualify for an extension in the future tell the bank you would like to open a fixed deposit account to hold those funds. If things go sideways, you can simply withdraw the funds anytime but you will have to forgo any interest. Hopefully the bank will also open a regular savings account at the same time.
If you're buying a new condo, ask the developer/seller if they can assist you to facilitate your purchase.