> Now the question: how often do these increase?

With respect to .io renewal on Gandi.net based on personal records:

  US$35.00/yr through 2018
  US$38.00/yr (+8.6%) into 2019
  US$42.18/yr (+11%) into 2021

If I recall correctly, .io renewal was limited to something like 1- or 2-year intervals not too long ago; this appears to have changed, but I'm unsure when that happened.