I had issues yesterday too. I filled everything out then after I received the email I discovered I messed up my middle name.
Filled out a new form but didn’t get a new email. I went to edit the new form a few times, saved it (without changing anything) and eventually got an email after about the third attempt!
Yes you can do it upon arrival, but I would try and get it done prior. There is a chance that kiosks set up to help will be busy with everyone else who forgot, didn’t know, or procrastinated and the last thing you want is another queue to join!
Sure you could use the airports wifi on your own device, but do you really want to risk a technical issue at that crucial moment?!
Chances are that at some point you will be sitting in an airport somewhere else in the world, waiting to board your flight to Thailand. Use that time as an opportunity to get the paperwork sorted if you haven’t done so already.
that’s what I’m referring to. They can’t let you fill out the form early because there are health related questions that you couldn’t possibly answer two or three weeks prior to arrival.