You didn't get off to a flying start, then you were asked specific direct questions to which you didn't give direct answers. This I found somewhat frustrating. However, let us consign that to the past and move on.
I have had one of these for a couple of years. I've been careful not to overload the inputs and if it started behaving as yours does, it would be returned to Siglent's main agents in the UK with a warranty claim. I find it intriguing the initial purchaser didn't do that, especially with so few hours on it, unless they knew it had been wrecked by overloading the SA and TG output. Maybe something else happened such as it failed and the original owner died, or it belonged to a company which collapsed before a warranty claim could be made. If you were confident it was a rare mixer failure, not caused by severe overloading, then returning it to Siglent to deal with would be the right thing to do.
I suggest you do exactly what tautech suggests. You just might get it working without opening it up and anyway, we'll know where we are.
If that runs dry and you have to open it up, let's take it from there and I'll help as best I can.
I've done some limited SMD work, and I certainly wouldn't like to tackle a faulty SSA3021X. For all I know you may have a top end SMD rework station and be a dab hand.