You can add arrow keys to the Terminal layout shortcuts: https://gitlab.gnome.org/World/Phosh/stevia/-/blob/main/doc/phosh-osk-stevia.rst#:~:text=TERMINAL%20SHORTCUTS
For example:
gsettings set mobi.phosh.osk.Terminal shortcuts "['Up', 'Down', 'Left', 'Right', '<ctrl>a', '<ctrl>e', '<ctrl>r']"
The full list is here (omit the GDK_KEY_KP_ prefix): https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gdk/gdkkeysyms.h
If you want to revert to the default, they are defined here: https://gitlab.gnome.org/World/Phosh/stevia/-/blob/main/data/mobi.phosh.osk.gschema.xml?ref_type=heads#L75
gsettings set mobi.phosh.osk.Terminal shortcuts "['<ctrl>', '<alt>', '<ctrl>r', 'Home', 'End', '<ctrl>w', '<alt>b', '<alt>f', '<ctrl>v', '<ctrl>c', '<ctrl><shift>v', '<ctrl><shift>c', 'Menu']"
Edit: The domain should be sm.puri.phosh.osk, not mobi.phosh.osk (for now).
gsettings set sm.puri.phosh.osk.Terminal shortcuts "['Up', 'Down', 'Left', 'Right', '<ctrl>a', '<ctrl>e', '<ctrl>r']"
gsettings set sm.puri.phosh.osk.Terminal shortcuts "['<ctrl>', '<alt>', '<ctrl>r', 'Home', 'End', '<ctrl>w', '<alt>b', '<alt>f', '<ctrl>v', '<ctrl>c', '<ctrl><shift>v', '<ctrl><shift>c', 'Menu']"