@dmitriy.ershov thanks for the response.
If you could just help us a little to get Ubuntu running on a Raspberry Pi (model 3 or 4) with Navio and Navio2 that would be highly appreciated. Do you think that would be possible?
On the other hand, if you plan to keep supporting Raspbian with ROS, please use the suggested ROS version for each of the Raspbian version, because the version installed with the lastest Raspbian doesn’t work (see RPI4 libboost 1.67 error - run custom ROS nodes - #26 by guido_sanchez).
Raspbian Stretch images should ship with ROS Melodic and Raspbian Buster images should ship with ROS Noetic. I am really not sure if installing the Ubuntu armhf is a good fit, I was having trouble while trying to compile ROS packages for ROS Kinetic on the lastest Raspbian image and I couldn’t update to Melodic or Noetic using the armhf Ubuntu packages (see Cross-compile ROS instead of using the Ubuntu pre-build packages by lucasgrelaud · Pull Request #8 · emlid/pi-gen-navio · GitHub) and the issues and pull requests of GitHub - emlid/pi-gen-navio. However, compiling ROS Noetic from sources worked on Raspbian Buster.