It looks like your visa will expire whilst you are in France, so you would need to start the process again, and would need to satisfy the online requirements to apply for the non O visa before coming to Thailand.
When in Thailand you would need to deposit 800,000 baht again for the first year extension. If you also make the monthly 65,000 baht deposits, for the second extension you can change.
I don't know if your Embassy in Thailand provides the income affidavit, if they do, you might be able to avoid the 800,000 baht deposit.
I don't think there's any other way, it is unfortunate as you are only out of the country for 2 months.
I would also call in at your local immigration office to ask for their list of requirements, as there are sometimes small differences between offices. You do not need an agent, the immigration officers are helpful and all you are doing is providing the information they request. Good luck.
each immigration office is slightly different, so the best thing to do is go to the office and ask for their list of requirements for the extension. It's usually; photocopies of all documents, passport, passport size photos, information from your bank, your lease agreement or tabian baan if you own a house, a hand drawn map of where you live. There's a 2 sided a4 form to complete with about 15 basic questions like; name, passport number, address, date entered Thailand. It costs 1,900 baht and on average takes me 45 minutes each year.
Lot's of people use agents legally because they perceive the visa process to be problematic or they might have health issues.
Some use it if they don't have (or want to tie up their money in Thailand), but this is illegal and people do get deported as a result. The rules are there for good reasons (to ensure people have enough money if they need medical treatment etc). If someone doesn't want to deposit 800,000, they can avoid it by applying for the non OA visa instead, this mandates they have health insurance instead of money in Thailand.
depends. If you are from a country that provides the affidavit, no. If you transfer 65,000 per month, no. If you are relying on the money in a Thai bank method, it can drop to 400,000 2 months after the extension is granted, but it must be topped back up to 800,000 3 months before the next annual extension.
Jon was stating an agent can help circumvent the 800,000 requirement for the first year, that is illegal. If the person is from a country that provides the affidavit, there's no need for 800,000 at all. For the second year you can switch to 65,000 per month transfers instead.