I've used Thai Visa Centre before, they were really excellent. I have no idea if that's who contacted you, or if this exact company does marketing in that manner. If you need an agent I would contact them directly, not through that email..in case they've hijacked the real place's system.
You can use a rapid test, but not a self-test, it has to be performed at a lab/hospital/clinic, with an official results document, and done within 24 hours of your departure. PCR can be done within 72 hours of departure.
No, not true, except maybe in a very local hospital? I've had surgery alone in an international hospital, no issue whatsoever. I was in a private room.
Centre Point brand hotels are really good. I stayed at Centre Point Sukhumvit 10, they are very responsive, but have heard they are all managed very well.
Your day 1 hotel should've given you the test for day 5, you do it yourself, take a photo then supposedly upload it to the app...but never really works..some people send it to the day 1 hotel, and they upload it into the system for you.
Yes, Centre Point in general is a great company. I used Centre Point Sukhumvit 10, also excellent. Booked directly with hotel, very responsive, everything great.