avocom aren't all Joomla components and extensions supposed to be written in database agnostic way using Joomla framework and its built-in BD management tools?
Yes, it should be. However, as you know our extensions were implemented very long time ago, to support PostgreSQL, we will need to revamp the queries so it can work with MySQL/MariaBD/PostgreSQL. I've shared with the team to update this but it will take time.
For now, if you want to test Joomla 5 + our template/extensions, please try to install it in local environment and see how it goes. Or consider to move to a new host.
About the system requirements in the document of template, as I mentioned in my above reply, when you install Joomla, then install template, it will work well with PostgreSQL, the issue comes from extensions though 🙂