Can't sprechen be used with accusative as well? Like in this example from the dictionary:
„Ich muss Sie unbedingt sprechen!"

„Mit”erfordert den Dativ; und ich habe eben gesehen, dass ich das versehentlich nicht verbessert hatte.
„Ich muss Sie sprechen!”, aber „Ich muss mit Ihnen sprechen!”

I thought mal and einmal were interchangable?

Doesn't this correction contradict the correction you made for me a couple days ago? I added the correction below, in which you said to capitalize the first letter after the colon.

„Meine persönlichen Schwerpunkte zum Thema Umwelt sind: Nicht zu oft zu reisen, auf Fleisch zu verzichten, die Lichte möglichst häufig auszuschalten und in Bezirken zu wohnen, in denen ich möglichst wenig mit dem Auto fahren muss."

In spoken language they are often used interchangeably, but your overall style suggests that that's not what you're after. This is a stylistic improvement.

Most of the time they convey a slightly different sentiment, too:
"Ich war einmal in Griechenland" vs "Ich war mal in Griechenland".

Hey thanks for all the corrections yet again! Do you just really enjoy helping people improve their German?
And thanks for the assessment of my skills. I've spent a long time reading and listening, so I can follow most of what I read/hear. But the truth is I don't know ANY Americans, or other English natives for that matter, who sound natural when they speak German. We all basically speak English with German words :/

No it doesn't ... feel free to look it up in the Duden ;)
If the statement following the colon isn't a complete sentence, but just a word cluster or interjection the first word isn't capitalised. If it's a complete, grammatically well-formed sentence it is capitalised.

I have met a few, British, American and one Australian native speaker who spoke fluent German (two of them with noticeable accents) but idiomatically correct - of course all of those had done the "full immersion" thing and lived in DE for years without having an English-speaking family around them.
And yes, I do enjoy correcting :)
It helps me with keeping my own German alive, too.

Ah I see! Many years in-country might be the only way to do it 🤷‍♂️

I also had one young English lady here partake in my German conversation group who was amazing; she didn't have a very big vocabulary, but she sounded awesome and had all the right expressions. I never asked her how she did it. She had German in school in the UK before she moved here, and kept at it at the "Goethe Institut" as well as attending my group. Saying: it *may* be possible even without full immersion =}