Hi all, I am in the same boat as my only OS is Ubuntu, and no other diary app can compare.
For those still interested, Waydroid works quite well with Diarium. Installing it per docs (I chose the Vanilla variant, as gapps require a lot of effort to set up), and then install diarium via apk in the terminal. Runs buttery smooth with near native performance. I have not tested every feature yet, but most things seem to work out of the box (I think there even is a way to map folders to the waydroid environment, so pictures can be used).
Disclaimer: This is only meant for freely and legally obtainable apks.
