"5 year visa" sounds like DTV, lasts 5 years of unlimited visits of up to 180 days at a time. Look up the requirements on your country's Thai embassy website to see if you would qualify
most are over 50. At the moment it doesn't matter what foreign income you have, it's what you bring into Thailand that counts (if you become tax resident). And if your country has a Dual Taxation Agreement with Thailand, the provisions of it will likely reduce or even eliminate the Thai tax liability. DTA's differ, so you need to refer to the one your country has. However if you're thinking of working remotely for your current employer you will need a different visa, like the DTA
Nope. You cannot generalise with DTA. US is very different to Australia, both very different to UK. I always advise people find the DTA for their country and read it carefully. Then they know about residency rules and double taxation provisions. It is pointless relying on Facebook groups because, like the comments in here, they are very misleading.