What are the customs regulations for bringing professional musical instruments into Thailand for personal use?

If you take it with you when traveling, it shouldn't be an issue as soon as it is not for work. If shipped, you'll most likely have to pay import taxes.

You shouldn't have problems, unless you are trying to bring a whole orchestra with you. Do expect to be asked about the kind of work you do if they do question you about it. Quite a few younger tourists bring musical instruments with them, guitars, and such. As long as they don't get the impression you'll be trying to make extra money on the side, you'll be fine.

The DTV visa is a tourist visa, and I think many people confuse it with the Elite Visa or the LTR visa. The DTV visa is not an official long-term visa. There could be problems if you, as a tourist, want to bring a professional instrument with you, because that raises questions. However, you could say that it's your hobby and you'd like to play music on vacation. Keep in mind when you go back to your homecountry with your instrument they might want custom fees/tax from you, because they will think you bought it in Thailand