As your firmware is based on “google marshmalow” android level (if it is latest available)
whatever existing /e/ version you choose, “n”, “o”, “p”, “q”, “r”, “s” will be right, even LOS 13, 14, 15, 16, 17, 18, 19, 20 All will be based on “mm” latest stock firmware
A contrario :
If the vendor OS is based on “google tiramisu” android level, you will be abble to install LOS 20 but not 19, 18, 17, 16, 15, 14, 13, nor /e/OS s, r, q, p, o, n, because none of them was based on a “t” stock firmware