with the multi entry tourist visa you have to follow the same rules as with a regular tourist visa (ie you get 60 days that can be extended another 30 days once. At that point you have to exit the country and re-enter, which you can do on the same visa (hence the multi entry bit). The visa itself is valid 6 months from date of issue. If you exit and then re-enter on its final day you then get 60 days which can again be extended for another 30, giving you a theoretical total of 270 days with 2 exits and re-entries. In practice it will likely be just under that as you're unlikely to enter the country on the day your visa has been approved
Over 50 and you can get 12 month "retirement visa". Lots of money to burn you can get Elite visa. Those are your only 12 month options. Tourist visa are 60 days and can be extended another 30. A multi entry tourist visa you can stretch to 9 months if you exit and reenter the country at the correct time. Other option would be to get an ED visa and take an educational course but you ability to travel around would be restricted by your classes
If you have an O-A visa you'll be stamped in for however long you're insured for so if you just want 9 months just buy 9 months insurance (you'll probably have to use Tune as I think most of the other providers only do 1 year policies but I may be wrong). The insurance for your whole stay must be purchased up front
Just bear in mind ed visas are only 90 days at a time and need to be renewed every 90 days (at a cost). If you're learning Thai you may well be expected to converse in Thai as a test that you're taking a legitimate course