You need to test your existing SIM. Buy a cheap full speed/max speed day pack and then walk around the spot you might choose to with; testing the speeds with https://testmy.net/ Choosing the Combined Test option. You are looking for relatively stable connectivity. Take a screenshot of each test. This should build a picture as to whether it will be a reliable option for you. I get good speeds whilst out and about with DTAC, so it should be fine.
It doesn’t need to be super fast, so an Unlimited pack may better suit if you’ll be online regularly.
A 6Mbps+ connection should suffice if the connection is stable. Of course the higher the better.
In the absence of any offers via the DTAC application, an Unlimited 8Mbps connection is 35 baht per day for test purpose. Unlimited 10Mbps is 482 baht for 30 days.