no problem, there is another route you can take if you are determined to come by visa exempt or tourist visa before applying for the DTV. You can contact an agent such as
just go direct for the DTV. Even with an SETV you could be pulled out of line, questioned and then denied entry. Its upto you what you do, but personally I wouldn't risk it as a visa doesn't guarantee you entry, it only increases your chances of being allowed to enter
You're flagged in the system and each time you enter now without a visa or even with a short-term visa your chance of denial increase 10 fold. I suggest looking at a more long-term visas before returning such as the DTV
It shouldn't be an issue, you might get questioned though. However... no one can say for sure as each IO is different and even then each person they see is different too.
Just make sure you have proof of accommodation, 20k in cash and a copy of your ticket out of thailand. If you really don't want to risk being denied you could always pay an agency like
that might be an issue with your phone or browser. Try clearing your cookies etc or a different browser. It could also even just be an issue with the website, they are notoriously known for not being very well made.
Regardless though, even on the very slim chance it is blocked, or there is an issue with the website, you have options Thai side, if worst comes to worst there are kiosks just before immigration you can do it on.
You do it in the airport on your phone or on one of the provided TDAC terminals before immigration. However I very much doubt China has blocked the TDAC website.