While I have never heard the name, "Bart Millard", in my life, I have heard of the group, 'MercyMe', and know the song, 'I Can Only Imagine' (great song), so in that sense, I sorta know who he is.
But anyway, I'm in the 'TBD' window, too, so I am hoping those get announced soon. Last time I got to see Jonathan Groff (Frozen's Kristoff, Broadway's Spring Awakening & Hamilton, Netflix's Mindhunter, HBO's Looking) and it just so happened I was a big fan of him, so I really lucked out, and he was truly a wonderful narrator, personable and friendly and warm, and it was his first time back in Disney since he was a kid, so his enthusiasm was palpable. (This was back in 2014)