Hello, Kat. There's low and there's low.
Some harps are available in Low F, like the Suzuki Manji you mention, and the Lee Oskar.
But there's also Low Low F, or Superlow F, and that's a whole octave lower than Low F. The Hohner Thunderbird, and the lowest Seydels (Session and Favorite) go this low. I own a Seydel Favorite this low, and am fond of it. In fact I named myself after it.
(You also named the Big River. It 's lowest tuning is G which is just a bit less low than Low F.)
Lo