Spirits/demons aren't two separate sides, they're beings of concentrated purpose/emotion. So the problems start when they're denied their purpose or they are forced against their purpose - then they become twisted.
So Wisdom doesn't contain an evil “Pride half” inside it but Wisdom can become Pride if it loses balance/denied it's purpose.
DA introduced the term demons but what Solas has been tying to explain is that they aren't separate things. Demons are spirits and spirits are demons - spirits will just exhibit the negative/corrupted nature when they can't fully express what they are.
So for me, yes I think Solas choosing to manifest changed him immediately (I believe Solas chose his own name as he was aware of this, but that's just me).
And pride isn't automatically evil. Pride can mean having a strong sense of self, believing in your convictions, caring deeply about something, or refusing to bow to what you think is wrong. The danger is when someone becomes too certain that only they are right. So if that spirit becomes completely convinced that its understanding is always correct, Wisdom can slowly turn into Pride. And if its constantly being ignored, dismissed, mistrusted, or forced into situations where nobody listens, that Wisdom can harden that way as well.
I don't think taking a physical body is what broke Solas. I think what broke him was constantly acting against his own nature afterward. Solas still feels connected to his spirit side (there's plenty of dialogue in DAI and DAV that supports this), but he spent centuries doing things that went against that nature: war, killing, tranquilizing the Titans, and making choices he knew were wrong or dangerous because he believed he had to for Mythal or for his people or for his own grief and regret.
Over time, continually ignoring his better judgement and turning away from the person he wanted to be slowly changed him (just like it changed all the ancient elves I'm sure).
So to me, spirits begin as pure concepts. Taking physical form creates emotional complexity, and that complexity creates inner conflict and contradiction.
I don't know, does this help? Make sense? It's just my own insight, I imagine others have way more succinct and amazing insight to share.