tl;dr - read this post
With a SF Symbol it’s Marker("Tues", systemImage: "4.alt.circle", coordinate: .marker)
With a one-to-three character monogram, it’s Marker("Tues", monogram: "+2", coordinate: .marker)
You can also use a Swiftui Label, though I can’t see how that’s any better / nicer than just the text and the SF Symbol.
To add a colour tint, add .tint(.yellow)
… doesn’t help make the marker tappable, though …
… but this might.