, If you are going to change to Non-B, you need to leave Thailand as Tod explained.
As you are on Non-O based on being married to a Thai, you can hold a work permit without changing your Non-O visa entry to Non-B.
BUT, it depends on your contract with your employer and how they would help with your visa/work permit if you need to change to Non-B or can stay on Non-O married to a Thai. Talk with them well.
Most immigration offices accept 1-year extension applications when you have 30 days or less with your stamp, so it would be 9 April. Some offices such as Bangkok, Chiang Mai, and Phuket, 45 days in advance. Check your local office for their policy.
You also have to have 800K baht in your account for two months minimum as of your application date. You sound to be fine on this point.
, You need to check with the local office for details. This is just a piece of general information, not the exact list at any of the immigration offices.
In case asked for photos for a retirement extension. Usually several photos of the applicant (with family if you have) in different locations and various positions, including one at the front of the facility (condo) showing the name and address, in front of your house number, in the dining area and others inside your house.
The bank letter is for the bank to prove this account (No. xxxxxxx) is yours (by name, passport number, etc).
That depends on your local office. Some allow online even for the first report, but some don't.
You need to check with them.
OR
Try online as soon as you are in the reporting window (starting 15 days before the due date) to see if it's approved. If rejected, you still have until 7 days after the due date to go to the local office to report 90 days in person.