I'm editing a story right now, and they use this construction with no speaking verbs a lot:
Rachel was finishing washing the dinner plates, “I know this will be hard on them, but Shelby needs to go on home."
I know you could replace that comma with a period, but I wonder if it's common to use a comma or semicolon to avoid slowing the reader too much?
This next one makes more sense to me because laughing could conceivably be a speaking verb:
Maddie's laugh was laced with sarcasm, “Mark, looks like you’ve got a friend.”
That one could even be a colon...
Am I overthinking this? Should I just replace them all with periods?
I'm of the objectively incorrect but subjectively extremely correct opinion that as long as the reader understands your intention, it's fine, and that no story is going to be made or broken by fretting about a comma or a period or a semicolon between a description of upcoming dialogue and the dialogue itself. This probably isn't helpful though.
I have a pet peeve about commas separating two complete clauses with no conjunction generally, and it bothers me every time I see it (unless it's two complete clauses in one line of dialogue, because people do talk like that, with short pauses). Such situations are often better with a semicolon or a period instead. But I have no idea if that is a common opinion. Above all, I don't want to steamroll over the writer's unique voice.
Not just "subjectively extremely correct", the reader understanding your intention is the literal definition of communication.
The problem as a writer, though, is that you don't know if the "as long as" is going to hold. Punctuation as a set of rules that must be followed is certainly unhelpful nonsense, but punctuation as a tool to help point those who might be misdirected without it is worth some consideration.