Haven't seen this one in a while. There was a covid insurance policy needed if you had to get tested in Thailand for your next destination (Mainly India and China), and if you tested positive, that means you had to either stay in Thailand, or go somewhere else.
But I think all of that is done away with in India, and maybe even China?
You could also take a side trip on the 45 day exemption (2 nights in Kuala Lumpur for example), then come back for another exemption period. Just a thought in case you were thinking of visiting another country.
You are good. As long as you have a flight booked to leave Thailand within the initial length of entry (45 days currently for visa exempt) before you board your flight to Thailand.
Also note that you are technically required to have 20,000 baht (or other currency equivalent) in cash in pocket. Immigration rarely ask, but it IS a requirement.
You can go for it now. Some places have different rules etc, but take your 1900 baht, take your form, passport and go now. Plus, it gives you time in case you forget something etc.
The 6 months starts at some point during application. Then you have multiple 60 day stamps (leave and come back) within that time frame. So, if you apply on March 30. Get it April 5th, you have until October 4th to enter. If you enter April 5th, you have to leave (or extend) by June 4th. Go somewhere, come back. Repeat.