Important: if you use a FCA (which I do), when you get your document with the balance, also get a statement of transactions, otherwise immigration will not accept it.
Perhaps the best value are the DTAC and True sims being sold, especially through Lazada, for around ฿1000, with free phone calls and free internet @4bps for one year.
Sure mate. The only reason (it seems to me) to use an agent for a visa extension is if you do not have the funds and they'll magic an illicit bank letter...
I've now now notched up over twenty retirement and marriage visa extensions.
Not one gram of 'stress'. In fact we're normally in and out of our immigration office in twenty minutes, probably less time than if you went to an agent.
Supposing at 50 you start using an agent and you're still in Thailand when you're say 70, that's ฿300,000-400,000 you've UNNECESSARILY wasted or over £8000.00.