That's the key to this whole discussion: "by my standards". Everyone has different measures of comfort and livability. I go out to eat almost every day and I don't consider the restaurants to be fancy but I do like my sushi! Travel is an important cost and with 3 trips per year back home plus side trips to Dubai, Singapore, Cambodia, and Vietnam then that's probably 4-5 times your amount. Omg, how can you live in such a wonderful international city with fantastic nightlife and not go to bars and clubs? Bars maybe twice per week, sky bars about 3 times per week, with some sporadic special events such as wine tastings. And I haven't even gotten back into my golf game yet!
I'm that single retiree you're referring to and I'm at~$3000 USD per month (and no girlfriend expenses), so I'd say you're about on target. $2000 budget would be a stretch in my view. Some will say you can live comfortably on half that, but just because you can doesn't mean you should. I came to Thailand to enjoy life, not to just get by.
Have you considered just waiting until November to come to Thailand? The 3 months will fly by. Certainly make use of an agent for both of you as it's the best way to optimize time (speaking from experience).
I understand where you're coming from as some agents are dodgy. Mine got me set up with a bank account where I then wired the necessary amount and I've been maintaining it with the proper balance for a couple years, and all completely under my name. So I think I'm in good shape.
Why glued to an agent forever? I can choose in subsequent years to use an agent or not. The stamp in my passport is the same regardless whether an agent was involved.