I get a bunch of identical messages "Notice: Trying to access array offset on value of type int in element_children() ..."
There are a few items here:

It's safe to upgrade CiviCRM from 5.26 to 5.30 in a single step.   As always, back up your database first, and if you need to restore it, it's safest to drop the existing contents before reimporting the backup.

Notices should not be configured to appear on a production site, they're mainly of concern to developers.  Since you use Drupal you can turn notices off from the UI.  Other should change their php.ini.


Regardless, notices shouldn't prevent an upgrade, and moreover, the notices appear to be coming from Drupal, not CiviCRM.
Finally - I noticed that MySQL 5.7.31 doesn't like to export data unless you a) grant all privileges, or b) pass --no-tablespaces to mysqldump.  Either approach is fine.