you don’t even need that. Just point your phone camera at it like you would any QR code. If your phone can read QR codes normally, it can read the Thai Pass one...
if you are entering 30 Day Visa Exempt (i.e. visa on arrival), you need to show proof of onward travel within 30 days. Even if you’re planning on extending it.
...if you are applying for a 60 Day Visa, get an outbound flight that falls within those 60 days. I'm sorry but I don't see what's so confusing about this. You are not trying to get a visa upon arrival, which yes, is for 30 days and thus has to show proof of onward travel within those 30 days. You are trying to get a 60 Day Visa. The 30 day stuff is irrelevant to you because you are not getting that kind of visa.
"or the 30 days and then change it" - Why? What would be the point in doing that? You're applying for a 60 Day Visa, obviously get an onward flight that reflects that. For what reason would they want proof of onward travel at 30 days when they've already approved you to stay up to 60...