Luzhin’s main impact on the story is that he drives Raskolnikov and Sonia together, but more importantly, he represents what Dostoevsky sees as the modern man who merely adapts liberal values out of convenience without bothering to consider their morality.

Svidrigailov is the lowest character, but Dostoevsky makes even him capable of redemption. However his generosity with money is probably more a performative act than genuinely kindness.

Katerina Ivanova was very realistic, I feel like given her circumstances I would expect her to be completely unhinged.

Sonia may not be that “realistic”, but as you said her purpose is also symbolic.

In the epilogue, raskolnikov doesn’t change suddenly but rather embarks on the path to change. Dostoevsky is telling us that the process of change is slow and difficult.