1) Embassy certified income. Many embassies do not offer this, so it's not an option for many people.
2) Monthly international transfers. You can do this by SWIFT from your home bank, or using WISE and selecting the funds for long-stay reason. Please note: This CANNOT be used for the first 1-year extension after converting to a non-O visa or entering Thailand with a non-O visa. Even if you have 10 years of international transfers you can show, it will not be accepted as you CANNOT use it for your first 1-year extension. You must either use option 1, or the 800,000 baht in a Thai bank account for the first 1-year extension.
How do you get confirmation from anything about immigration? You go there and ask. Each immigration office has their own policies, so something someone tells you might not apply to you because maybe they use a different office.
This is not a question that has an answer. It's as long as the immigration officer you stand in front of when entering Thailand thinks is enough. Every officer has their own idea of what's enough and what isn't. If you are doubting whether you are out long enough, then maybe you aren't.
They will want both your entry stamp and your TM30 as part of the application to get a CoR, and they'll likely notice your TM30 is from before you entered Thailand and it will not be valid.