Best option is buying an empty SIM card from a shop for like 20-40 baht then downloading the carriers app and buying the package that way. The carriers shops have limited options and in my experience they try and sell you the most overpriced things and intentionally hide lower priced options. I recommend AIS with the MyAIS app.
They do a colorblindness test. I have mild colorblindness that's not noticeable and doesn't interfere with me at all, but it makes it so that I can't pass that test. Sucks.
Mine took 13 days, but I think they may have been having issues with their system when I applied in November. Should leave plenty of time to be safe :)