Is there a difference between "I guess" and "I'm guessing"?

The OED does provide a separate definition for I guess; It is definition 6:

I guess: sometimes used, with playful moderation of statement, in reference to what the speaker regards as a fact or a secure inference. Hence colloq., orig. in the northern U.S. (sometimes with omission of the pronoun) = ‘I am pretty sure’.

The three earliest attested uses are from texts written by John Locke, suggesting that he might have popularized the sense with this kind of meaning. The earliest citation is from Some Thoughts Concerning Education, dated 1692, so it's not an incredibly recent idiomatic turn.

Hence, your intuition was correct when you wrote:

The impression I've gotten as a native speaker is that "I guess" doesn't literally mean "I guess"; it means something more like "I conclude".

As a final note, I guess isn't limited to this sense of the word implying something close to certainty. It could still be used in a phrase that meant an actual guess, but such a use would be distinguishable through context.

Jim: "I guess there are 150 jellybeans in the jar." (sense 1: a real guess)

Sally: "I counted, and there are 287 jellybeans."

Jim: "Oh. I guess I was wrong." (sense 6)

As you can probably imagine, had Jim replied with the present continuous tense of "guess," it would have seemed strange.

Jim: "Oh. I'm guessing I was wrong."

Sally (confused): "Yeah, you were definitely wrong."

This seems to be exactly the distinction you described in the question, so the answer is yes, you're on the mark.

you could be 'seeing' or picking up on a subtle distinction between the two because of their tenses.

i guess

is a present simple tense

but

i'm guessing

is a present continuous tense.

the difference between the two is that a present simple tense describes a fixed/unchanging situation while a present continuous tense describes a temporary/unfinished situation:

present simple: the speaker is portraying that they're not sure about their guess and not necessarily willing to change their guess.

i guess... (I thought about it just now, and I concluded that it would make the airplane hard to control. I'm pretty confident in my conclusion, but I'm not sure that that's the actual reason why you shouldn't press that button).

present continuous: the speaker is portraying that they're not sure about it and may like to change their guess (with more information).

i'm guessing... (My best guess is that it's because it would make the airplane hard to control. Is that right — would it actually make the airplane hard to control? And is that actually the reason why you shouldn't press the button?)

if i too am seeing a distinction where none exists then i guess someone else will provide a better answer.

i'm guessing they could also cast a downvote...because they disagree.