Is it common to say "first off" or "first of all", but there's only thing the speaker has to say?

I would say not uncommon but it sounds strange to me. I feel like you would only say that if you thought you had more to say than you actually did

I think often they're using it to make it sound like what they're saying is the most obvious of a list of things. So before even considering the other things, you should acknowledge this big thing. Implying that what they're saying is just one problem out of a multitude of problems, for example.

If I start with one of those two phrases but do not follow up with a second item/point, it’s almost always intended as a joke, or to be sarcastic/skarky Otherwise, I will add my additional points with the use of “then” or “secondly,” etc etc until all my points are made.

I think it's pretty common as a "now listen to this, this is important" thing. It may be intended that it's the first thing you should consider, but it may imply that there are other things but the person simply isn't going into them.

It's common. But it's usually followed up with another thing in non list format. So "first of all, blah blah blah. Also, blah blah." Sorry I couldn't think of specific examples

It's become a kind of internet meme to say "First of all" followed by an insult, sometimes a little childish. Example: [https://poorlydrawnlines.com/comic/discussion/](https://poorlydrawnlines.com/comic/discussion/)

I hear that from Sean Strickland all the time

I hear "first up" more than I hear "first off": they both mean "first" or "firstly".

I think it's fairly common. When you say "first off" or "first of all," you're not necessarily setting up a list. You're just saying that the first thing you say is the most important or is foundational for the rest of the things you're going to say.

If someone is speaking off the cuff, or in conversation, I can see how they might say that and then get off track and never get to the second thing. But this shouldn't be used to open a formal presentation if there's only one point to make.

No, it's not common and pedantically speaking it's wrong, but people are not always precise. It's not so bad as long as you know what they mean. Just don't get me started on the word "literally".

I would never use it unless there was a second thing I wanted to say. However, I might start off with, "For one thing, ..."

There’s also that tendancy to add "et cetera, et cetera, et cetera" when you’ve completely run out of examples.

My ex would catch me on this, and also mixing up verbal list items. As in “number one, blah blah blah. And B, blah blah blah”.

It definitely sounds unusual. It might be common in an argument, where a person intends to say more than one thing, but before they finish their 'list', their first point is replied to by the other person.

This is a similar case to people who follow an "on the other hand" with another "on the other hand". Sometimes constructions that have specific forms in the original usage become reduced to simple pacing phrases without reference to an exact meaning.

My mom: First of all, you forgot to take the chicken out of the freezer. Me: And second? My mom: I'll give you a second!

I would view it as "This is so urgent to me, I want you to think of it as top of the list", rather than literally "there are more things coming after it as well". Like, someone who doesn't use "Primarily- " or "First, and foremost- " much, but wants to get the same sense.