Many countries have the rule that you can only apply from somewhere you have residence (can be different to your passport country). But I'm pretty sure Thailand don't have this rule - although it could be down to the individual embassy to decide
The STV is not so good if your plans are in any way flexible, because you have to book the entire 3 months accommodation before applying for the visa. Also since they're discontinuing it in September, it could be that they refuse renewals after then too.
Yup, TR is 60 days, visa exempt currently 45. It says you can only extend once but at the moment while the borders are closed they're letting people extend indefinitely
This is not true. Thailand do not have this rule - you can apply for a visa at any Thai Embassy or consulate - it doesn't have to be in your country of residence.
I applied for the TR visa because I was worried about the airline telling me my return flight was too late. But for an extra 15 days vs visa exempt it does seem like a lot of money and hassle for not much benefit.