You need a COE (certificate of entry) from the embassy in Manila, and yes you need $100,000 usd covid insurance policy, not money in the bank. Check the Thai embassy in Manila website for requirements.
I used to fly Cathay or Eva, but the last time I went with Jal and was really impressed by how polite and caring the cabin crew was, plus the plane was newer and wide comfortable seats in economy. Jal is my new go-to airline, highly recommended.
You don't even have to do it yourself, many agencies will take care of everything for you, for around 200 pounds, as long as you have the 800k in a Thai bank. If your over 50 it will work, no worries.
I think the stamp is good for 3 months but the Visa should be good for 15. 2 different things. But really it's okay either way I'm not trying to cause an argument.