Try the cli way:
https://wiki.archlinux.org/title/Graphics_tablet#Remapping_buttons
Find the device name
$ xsetwacom list devices
Remap Button 1 of the tablet to Ctrl++
, for example.
$ xsetwacom set 'device name of the pad' Button 1 "key +ctrl + -ctrl"
Read the Permanent configuration section of the ArchWiki article for how to keep the settings between restarts.