My advice would be to skip the Non-OA visa and enter on a single entry Non-O visa based on being over 50, transfer the 800K THB to a Thai bank account, and then apply for a one year extension of stay based on being over 50. Health insurance is not required for that extension of stay if entered on a Non-O visa.
If you're rejected for a tourist visa, it will have NO impact on entering on a visa exempt (or visa on arrival, or bilateral agreement, depending on nationality).
Looks like there is some slight confusion happening in how the process works:
>My question is I want to cancel my work permit and visa by myself.
Cancelling work permits is outside the scope of the group, and depends on the local labor department. You *MAY* need the employer's support to get the work permit cancelled.
After the work permit is cancelled, you will then go to the immigration office and cancel your current one year extension of stay based on employment.
> I need to cancel my visa with my current company and need to extend 7 days to wait for new one with my new employer
There is no 7 day extension of stay; there is a 7 day extension of stay denied stamp. Once you receive this stamp, you MUST leave Thailand.
How it should work is on the SAME DAY that you cancel your existing one year extension of stay based on employment your new employer will help you get a NEW one year extension of stay based on employment
If you have a half Thai child, another option is a one year extension of stay based on raising half Thai child. There is no seasoning requirements for the money.