Can I use funds gifted from my boyfriend to meet the financial requirements for a Destination Thailand Visa (DTV)?

Check with the consulate or website you are applying at for the length of time they want to see the funds in your account, it varies from a day to 3 months to 6 months.

If you are applying through NYC USA consulate they required a 6 month course FYI.

Some consulates accept shorter courses

You don't need to provide anything other than a bank statement in your name with 500k thb. Keep it simple

I don’t think a letter is needed. The Embassy does not need to know where the money came from. Not my personal experience, I read on some posts here (if you search with a keyword “500,000” or funds) people transfer the required amount from a brokerage account to their main account days before a monthly statement is issued so when they download the monthly statement the ending balance would show 500K baht. Personally, I submitted 3 monthly statements showing the required amount. Each embassy has their own interpretation of the requirement but to be safe, I would submit 3 monthly statements showing the required balance. Btw in my case I did not show the details, just a screenshot of the summary page. Good luck!