There can be multiple factors, with multiple effects. It's complicated. Some of it is contextual. As I said, an important part is expectation, and the implicature generated by not saying any of various possible alternatives. The effect may be different in different contexts, and for different reasons.
As for these specific examples, yes, the progressive form is "less definite", so that could mean various things: casual/uncertain, or also polite by being unimposing (because it's less definite/demanding). The simple present would sound a bit more formal, and possibly therefore polite, but also could sound more direct and therefore rude. As I said, it's complicated!
I suggest trying to understand relevant factors and their possible effects, instead of trying to determine "rules" about usage.
(Amusingly, I just noticed that, after correcting a typo, if I had ended this with "I am suggesting" vs. "I suggest", the progressive form could actually sound impolite, somewhat insistent, like "I've been trying to tell you for a long time....", while the simple form sounds like a new idea, and therefore not impolite. Other times, as you've noticed, the opposite is true.)