SUDO_EDITOR=kate sudoedit /etc/sudoers

to run Kate as root

but how to set up txt file associations , say with .txt files to do it ?

it used to work for ages, until some security guru came along and decided that 100's of thousands of users can no longer do it - all of a sudden.

in sudoers, to make life easier you'd do what all these gurus freak out about:

## User privilege specification
##
root ALL=(ALL) ALL

## Uncomment to allow members of group wheel to execute any command
%wheel ALL=(ALL) ALL

## Same thing without a password
%wheel ALL=(ALL) NOPASSWD: ALL

## Uncomment to allow members of group sudo to execute any command
%sudo ALL=(ALL) ALL

## Uncomment to allow any user to run sudo if they know the password
## of the user they are running the command as (root by default).
Defaults targetpw # Ask for the password of the target user
# ALL ALL=(ALL) ALL # WARNING: only use this together with 'Defaults targetpw'

but how to set up txt file associations , say with .txt files to do it ?

I guess you do not need permanent association of all files with "root kate", but want to easily edit individual files without typing a password at each saving.

For this I have found a workaround solution, see here: https://unix.stackexchange.com/a/655572/264065