humlan Hello
The Quickstart package is the same with Joomla package but with sample data as our demo site so if your site already having content now, you should not use this Quickstart package.
If your site is new, you can use Quickstart package and create new database for it - its the same as you build a new website.
T3 framework is compatible with Joomla 4, you can share the error in details here and super admin account of your site so I can have a closer look and check for you.
Regards