I actually think any of your favorites except Leo go with Leland. I esp like the sound of a 3 syllable fn with the emphasis on 2nd syllable like Matthias or Josiah or Isaiah. Matthias Leland sounds good to me.
I agree. I think you are overthinking this, Mag! Leland sounds like an excellent middle name with any of those names listed except Leo and maybe Graham. Meaning always trumps flow in my world but I don't even think the flow is bad!
Matthias Leland Parker (I dunno your last name).
Henry Leland Parker
Josiah Leland Parker
Jasper Leland Parker
They all sound so distinguished!