With modern version of Visual Studio, if you're interested in the languages services, then you don't have to use DTE and the old FileCodeModel stuff, but you can leverage The .NET Compiler Platform SDK (aka Roslyn) which is the parsing backbone now.

So, first thing to do is add the latest Microsoft.CodeAnalysis.CSharp.Workspaces nuget package (roslyn for C#) and the Microsoft.VisualStudio.LanguageServices nuget (Visual Studio Roslyn workspace) to your package project. Note you may have to fix the now-usual nuget mess...

Once this is done, you can write this kind of code instead of yours:

// get component model & Visual Studio Roslyn workspace
var componentModel = await package.GetServiceAsync<SComponentModel, IComponentModel>();
var workspace = componentModel.GetService<VisualStudioWorkspace>(); // requires "Microsoft.VisualStudio.LanguageServices" nuget package

// enum all the projects
foreach (var project in workspace.CurrentSolution.Projects)
{
    // enum all the documents in the project
    foreach (var doc in project.Documents)
    {
        // get the semantic model & syntax tree root
        var model = await doc.GetSemanticModelAsync();
        var root = await model.SyntaxTree.GetRootAsync();

        // find a class named "TestBase"
        // ClassDeclarationSyntax etc. requires "Microsoft.CodeAnalysis.CSharp.Workspaces" nuget package
        var myClass = root.DescendantNodes()
            .OfType<ClassDeclarationSyntax>()
            .FirstOrDefault(c => c.Identifier.Text == "TestBase");
        if (myClass != null)
        {
            // find a method named "AddItem"
            var myMethod = myClass.Members.Where(m => m.Kind() == SyntaxKind.MethodDeclaration)
                .OfType<MethodDeclarationSyntax>()
                .FirstOrDefault(m => m.Identifier.Text == "AddItem");
            if (myMethod != null)
            {
                // get the list of method parameters
                var parameters = myMethod.ParameterList.Parameters;
                ...

                // get the start line for the method declaration
                var lineSpan = model.SyntaxTree.GetLineSpan(myMethod.Span);
                int startLine = lineSpan.StartLinePosition.Line;
                ...
            }
        }
    }
}