You also need a letter saying you can work remotely and specifically in Thailand. You could also add that you will not be working with Thai people or Thai companies or organisations.
The TM30 has to be completed within 24 hours of you arriving at accommodation. The landlord or owner of the property has to do this. Not the tenant of guest. They do this online
I am thinking along same lines as you are. I had to cancel "O" to get "B". There was no "Converting". As for cancelling DTV that is an interesting point. When I wanted to cancel my "O" they would not do it at BKK - they told me to go back to issuing office ie Jomtien. If the same is applied to DTV this might cause some inconvenience if anyone wanted to cancel.
Is there any guidance on converting DTV to another visa? I have not seen it in writing anywhere. My experience with a Retirement Visa was it had to be cancelled before I could get a "B" for a WP. There was no "Converting"