document library – Filtering lookup values based on another column powerapps

I have a document library (with document sets). And while I wanted to use enterprise metatags (so I can use nested tags) the users feel that this is too much of a confusion and would like to have two columns where the second would be filtered based on the first. I thought of using powerapps and followed plenty of examples like this one
but it seems like an overkill to have two sharepoint lists while I could have one.
I found this topic: Filtering lookup values based on another column in another list, sharepoint online and powerapps but I can’t decypher what is the logic behind the solution.
and if I just try to copy and paste (replacing names) I get constantly errors.

In my scenario, I have a sharepoint list called “Project Tags” with two columns. One is Document Category and the second Document Type with a 1:N relation.
Then I have the doc library where I thought of using a drop-down to represent the document categories and a lookup for the document type.

Now what I look to do is to filter the document type, based on the value of the document category. If needed, I can replace the drop-down with a lookup.

Anyone kind enough to help me out with this?