every question you are asking depends on the Thai embassy you use. There are no formal rules written down. Right now the shortest, and cheapest place to apply, and wait, is Yangon in Myanmar, at about three days from last reported. It is real cheap to wait there, and it is not dangerous at all.
You can not use your monthly bank statements. You must get a deposit summary from your bank showing all 12 months of deposits coming from overseas, and the bank letter stating it is your account. That is the only thing immigration accepts if you don't have an embassy letter. Also be aware that you still must show that your bank balance never dropped below bt400,000 for the time until you apply for your next extension, even though you are changing to the monthly payment method. It was required for your last year's extension.
You can not extend it from outside the country. After it expires just apply at the Thai embassy, as if you were starting over, getting the 90 day visa first, then the year extension after you return to Thailand.
From what I've seen no income from your own LLC shouldn't be a problem, as long as it is a legitimate business, filing taxes. Some embassies might ask for your latest personal income tax filings, when your LLC isn't making enough money to pay you. Other than that, just a letter from the LLC stating that you can work remotely.
corrupt immigration agents, and those get cracked down on. The last was after covid with senior immigration agents from three provinces were arrested, and 138 "students" were banned.
In order to use the monthly deposits method, your embassy must issue a notarized wage/pension verification letter, and the UK embassy no longer does that. That is the only thing immigration accepts the first year. You can not get an OA visa inside Thailand. Your your situation you might look at the soft power options of the brand new DTV, destination Thailand Visa. It is a five year multi-entry visa. You get 180 day entry stamp on each entry for the next five years. Under that visa your wife can get a trailing spouse visa. Something to think about, because your current plan is not doable. For the DTV you do need bt500,000 in your bank account, anywhere in the world. You exit Thailand and apply for it with the e-visa system in a neighboring country. You can't get it inside Thailand.
actually it is true. Online language classes do not qualify for an ED visa. You can take online classes anywhere in the world. You don't need to be in Thailand. During covid online classes were allowed, but that ended a long time ago. Anyone professing to offer an ED visa for online classes is a scam, and with the crackdowns it can get you banned from Thailand.