Great post. And welcome to the forum.
I've never liked the definition of social anxiety as an intense fear of embarrassment. For me it's the intense fear of being seen as I truly am.
I think SAD is better understood as a particular strategy for dealing with being seen. (In this case, not being seen.)
I think that for a person to be mentally healthy, they need at least two things: they need to be seen as they truly are, and who they truly are has to meet with acceptance (by at least one other person). The first is necessary because it's only when we are seen as we truly are that we can experience a real connection with another person. It's the only way we can avoid feeling isolated and alone. Being seen as something that we aren't (being misunderstood) fails to create this connection, which is why you can be alone in a crowd. You can know a lot of people and still be invisible. If no one at all knows who you are, you can start to feel unreal and like you don't even exist. This is why this:
is the basis of all romance, and partly why romance has become a "religion" in our culture.
I would go so far as to say that it is better to be seen as you truly are, even if what you truly are doesn't meet with approval, than to be invisible. It is better, from a mental health standpoint, to be known and disapproved of than to not be known at all. But it takes some people a long time to figure that out. I know this issue from the inside, as it were, because this is exactly the predicament LGBTQ people are in. You can only stay in the closet for so long before being in the closet becomes toxic for your mental health. At some point, almost everyone in the closet snaps and "comes out", because they would rather be seen and hated than invisible and completely cut off from everyone else. (Ofc, being alive is even more important than being seen, so people will stay in the closet if they think coming out will lead to actual death.)
I don't think a person can be healthy or happy if no one sees who they truly are, so hiding from other people isn't a solution. That's why people come to a place like this to tell the truth about themselves. They wouldn't do that if there wasn't a very strong psychological need. They just wouldn't bother to say anything. What they're looking for, ofc, is acceptance. They want to be understood. They want people to say it's okay for them to be them. Because if they can find even one person who thinks it's okay for them to be exactly what they are, it makes it possible for them to give themselves permission to be exactly what they are. And you can only be mentally healthy when you are capable of being exactly what you are and being okay with it.
Exposing yourself on an anonymous forum (as you have done) helps meet this need, but it isn't terribly satisfying, either. It's like trying to live on nothing but bread and water. For real health and happiness, I think people need at least one real relationship IRL with a person who knows and accepts them. I think there are probably physiological reasons for this. This is why therapy (with a good therapist) helps people. A therapist is a person you can tell your truth to who will accept you as you are. (That's the ideal, anyway.) Unfortunately, it takes a long time for most people to develop a real relationship with a therapist, so most therapy doesn't provide this kind of benefit. (You won't get it from brief CBT therapy, for example.) This is also why support groups benefit many people.
This is also why having a true friend is so important. Most people don't need to go to therapists because they know at least one person they can be (at least mostly) honest with (for a lot of people, especially men, that's their spouse). Being seen by that person, and being accepted, allows them to accept themselves, and that self-acceptance helps them avoid developing psychological disorders. This is partly why you see a strong correlation between social isolation and mental illness. This is also why people can spontaneously overcome disorders without therapy. They just manage to meet someone IRL who could provide that support. (This all ties back in to attachment theory.)
Ofc, this is a gross simplification, and it's all actually quite tricky in practice. For example, a lot of people have very distorted self-concepts, and it's often necessary to work through those distortions before they give themselves permission to be themselves. Just being seen and accepted isn't always enough on its own.