Just sharing this experience for posterity. I ended up having to switch to Pop!_OS 22.04 LTS, because Ubuntu MATE did not have a working On-Screen Keyboard (OSK) for use with touch-screens running Ignition Perspective (Linux). The install steps for Ignition were exactly the same as above, since Pop!_OS is also based on Ubuntu. It's also similarly lightweight.
When manually searching for the gateway on the Perspective Launcher, the OSK was not working, so I had to use a keyboard for the initial connection to my project. However, once I entered the perspective session for the project, the OSK was working again (for login, textboxes, etc.).
I also wanted to enable auto-login to the OS, in the event of a power outage at the plant. There is a toggle in System > Users for auto-login. However, when this is enabled, the perspective session launch will trigger a popup that says "The login keyring did not get unlocked when you logged into your computer". This popup will prevent interacting with the Perspective Session in KIOSK mode until the popup is cleared. So I ended up going to "Passwords and Keys", right-clicking the login to change the keyring password, and set the keyring password to blank (didn't type anything, just pressed enter). I figured this is OK since there is no sensitive information stored on the OS, and Ignition already has its own login security.