Apparently, I value my time more highly than you value yours.
It's common to wait hours in line at the Immigration office, even for 90-day reports - in hot sun or rain. Immigration officers often speak English poorly and are rude to expats. If you get one thing wrong with your application, you have leave, fix it, then come back and restart at the end of the line. It's unpleasant when an Immigration officer asks for a bribe/tip, which happens. I value my time and can easily afford the visa service fee. I don't worry about the small amounts that seem to thrill you.
Not sure what you're talking about 400K. I didn't say anything about that.
In Pattaya, expats can and do switch between visa agencies. It's a competitive business environment. Switching around is what keeps the visa fees low and similar across agencies.
I've been visiting Thailand for +20 years. The older I get the less I like dealing with immigration. Used a local visa agency for the first time in Pattaya last year. Easiest most pleasant visa experience I ever had by far. Reasonable price. I highly recommend it.
It might be worth contacting a visa agency in your area. Hopefully you can find one with good Google reviews. These agencies know how to work with the local Immigration office and might be able to streamline getting a retirement or marriage visa. It's worth a call. In my experience in Pattaya, the agency fee was fair and affordable.
When I arrived in Pattaya last year, I dreaded applying for a retirement visa directly, because I don't like bureaucracy and think the official visa instructions are ambiguous at best. Fortunately, other retired expats told me how easy retirement visas are from local visa agency. Two services were mention often: Maneerat Service and Cambodia Inter Tour. Both have good 4-star Google reviews and are in central Pattaya. After visiting each, I choose Maneerat. It cost ~$800 for a 1st time retirement visa (lasts for 15 months). Included bank setup and multi-entry visa. (Costs ~50% less to renew it after the 1st year.) The visa service was very easy and stress free. Well worth it not having to deal with the immigration office myself. They also make the required 90-day reports easy.
I wouldn't schedule on a Thai holiday if I could avoid it. If I couldn't avoid it, I'd try to call/email the embassy to confirm. It doesn't matter what anyone on Facebook says. It only matters what the Thai embassy staff say. Good luck.
If I was doing this, I wouldn't schedule for a date that may or may not be an embassy holiday. Why risk it?
Another tip: Arrive at the Thai embassy as early as possible (7am sounds good). The line can be very long. The line doesn't move fast. They may stop processing applications for the day at lunch time.