How would it be to do this job of bringing hestia and debian up to date?

Step 1: Update Hestia to the last current version (Debian 9 is still supported)
Step 2: Now you have 2 options:

Debian 10 no changes are required
Debian 11: Follow the instructions

You also might need to upgrade MariaDB and maybe even switch to php-fpm depending on your setup