I think a line like that would work well! It's simple and gives the player that instant feedback.
I'd be careful implementing something like a slower walk. The idea makes sense on paper but tends to annoy players in practice. But you could do a 'weaker' looking walk animation, perhaps. And don't underestimate how the gameplay itself makes the player feel; being unable to reach spots that seem within my reach already makes me feel weak, or having to avoid things that insta-kill because I have nothing to protect myself. And picking up abilities that now let me do those things already flips that feeling around, with or without a 'weaker' walking cycle.
The way I handled the text is: press Space once, it skips the dynamic typing process by writing out the full text, press Space again, it moves on to the next bit. It checks if the writing loop has finished, which happens either through naturally letting the text write out, or by pressing Space the first time. I can use the same button for both functions that way, and I wouldn't worry too much about people skipping text by accident, since these are common text mechanics in virtually all games.
Yeah, the interaction with the Slime feels better to me, because I'm the one who initiated it. Whereas a forced cut scene feels like having control stripped away, something that I prefer games avoid where they can.