If I may give some advises.
Upgrading from Tiki 9 to Tiki 21 will be quite an adventure in one shot. They are also things about database, encoding, etc. that have been changed.
If you have a proper environment (like a Cpanel that allow PHP version selection) I would try to upgrade LTS by LTS. (9 => 12 => 15 => 18 => 21 ...)
At each step you should be able to fix little things that will allow the next step without errors or warnings. You may also to work on the database itself around Tiki18-21 (like encoding and InnoDB conversion).
Thanks to many people works and investment in Tiki from Tiki15 and up you will have more and more option to solve and get reports/logs.
Finally at Tiki21 you will be able to use the conversion script for Image Gallery => File Gallery OR to download your 20-30 images and simply batch upload them into your new Tiki.
Good luck and let us know how it goes.