yes, that would definitely be much easier Marc. You can do it 30 days in advance and the time is added on to your current extension, so there's nothing to lose. Don't forget to get a re-entry permit at the same time. Good luck.
I am unsure what the requirements are to get the non O in France, look online at the Embassy website. The only way your monthly pension will be accepted for the 12 month extension is if the France Embassy in Thailand will provide you with an income affidavit. If not, yes you will have to deposit 800,000 in your Thai bank account immediately. The 65,000 per month transfers only works for the second and subsequent annual extensions.
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.