Extending Visual Studio 2017 XML editor - visual-studio

Question

I'm trying to improve our workflow with some XML-configuration files we use in a Visual Studio Solution. I figured I'll start with explaining what I'm trying to accomplish and then explain some of my ideas that I've tried for several hours without success.
The Problem
I'm working with a system where the data model is configured in a database, the models are also stored on disk using xml-files, these are then synced during startup of the app. Here's part of the stuff that are synced:
Data Types
All data types lives in one XML configuration file per data type and each has some properties like Name and Key. All data types are configured in the same folder, example: /config/dataTypes
Example
textString.config
<?xml version="1.0" encoding="UTF-8" ?>
<DataType Key="a45c9a94-09d7-4df9-85e6-d6930abc6c12" Name="Textstring">
textArea.config
<?xml version="1.0" encoding="UTF-8" ?>
<DataType Key="b104788f-4e2f-4592-9387-8b3861bd8046" Name="Textarea">
Entity Types
Think of these as models with X number of properties, each property is configured and connected to a Data Type.
Example
customer.config
<?xml version="1.0" encoding="UTF-8" ?>
<Entity>
<Properties>
<Property Name="Firstname" TypeKey="a45c9a94-09d7-4df9-85e6-d6930abc6c12" />
<Property Name="Lastname" TypeKey="a45c9a94-09d7-4df9-85e6-d6930abc6c12" />
<Property Name="Description" TypeKey="b104788f-4e2f-4592-9387-8b3861bd8046" />
</Properties>
</Entity>
My end goal is to provide intellisense when editing a Entity Type so that I'll get all the Data Types as suggestions when entering the "TypeKey" property of the Property element. Preferably showing the Name of the data type in the dropdown list and when choosing it adding the guid as value for the TypeKey-property. I would also like to show the Data Types name when hovering the GUID with the mouse (or in some other way).
As you can imagen during the process with a project new data types can be added so the intellisense needs to be dynamic based on the files in the data types-folder.
I should also mention that these configuration files also can be edited from a UI in the application and I don't have control over how they are serialized, hence - there is no XML namespace in the files, they look like in the examples above.
We also have multiple different VS-solutions with different setups of Data Types and Entity Types. So we need individual intellisense in each project.
What I've tried
My first idea was to create a VS-extension that creates a dynamic XML Schema XSD-file based on the files in the data types folder. The VS would then pick up and use the XSD for XML-files in the Project, turns out that this only works with a namespace configured (remember, I can't add namespaces since I don't control the external app).
To get around this it worked to manually add the schema-file to the XML file while editing the file. (Like this: https://learn.microsoft.com/en-us/visualstudio/xml-tools/how-to-select-the-xml-schemas-to-use?view=vs-2019). The problem with this is that it needs to be done for each xml-file and the setting is not stored.
I then started to play with the VS extension possibilities to try to automatically configure this Schema to a XML-file that is opened. Turns out I had a really hard time to get the "configuration object" that is pushed to the Properties Browser when the XML file is opened. I managed to get a event-handler setup that was triggered when the XML files as getting "focus" but I was stucked trying getting the properties from the Properties Browser.
I also tried another approach with a custom "code completion"-extension but it made the UI feel very "cluttered" while editing.
After spending about 10 hours playing around with this I feel I don't really have a good path to a solution as my experience with VS-extensions is not that extensive.
I would love to get some feedback and ideas on how to approach this problem, since the XSD is different from each project if I go down the "dynamic" path we can't use the global VS settings since each project has different data types hence needs different allowed values for the TypeKey property.
One approach that I'm thinking about is to use a generic XSD (allowing all values for the TypeKey) in the Global Schema Cache for Visual Studio and create a "code completion"-feature for just the TypeKeys.
Any thoughts and pointers in the right direction would be very much appreciated.