None, just use layers. There are 4 keys with a frequency of 0.15% or lower: https://en.wikipedia.org/wiki/Letter_frequency For a text of 1000 symbols, on average, that would be less than 5 of those characters total.
That being said, the requirements for your muscle memory increases, yes.
For those that don't know what Layers are - please google the concept yourself; if I try to write it spontaneously in my own words, the result will be subpar.