Project parameters and package variables

Question

Hi all,
I am trying to complete an SSIS project that has three packages.
The end goal is to have the project deployed to the SSIS Catalog and use the appropriate configuration for a few different environments when setting up Agent Jobs on said environments.

My setup is as follows:
I created a Project parameter for the Database Connection.
I created a Project parameter for the path to three different input files.
I created a Package variable that holds the name of the file.

What I am trying to do is have the ability to set up the name of the server for the Database Connection (or rather the whole Connection String when configuring),
and the same for the input file path.
What is different among the three packages is the name of the file.

However, when going back to the (project)flatFileConnection which does have the hardcoded file name appended, I cannot express this connection string using the project parameter and adding the package variable, so that I have something like @[$Project::flatFilePath_ConnStr] + @[$Package::varFileName].

I guess I can hardcode the name of the file, but it is a way to combine project params and package variables?

I am sure I am missing something but I can't figure it out.

Thanks,
elsvieta