Electrons can not be any more unfriendly than they are to me... I hate electricity....
The good news is this is a mechanical project given your symptoms...
The shop manual does not show how to fix the starter.... I have looked... and put off putting mine back into my 240 because my backup starter off a 300 I may shortly need to power my new DC welder...
I feel I need to explain why I did not tackle the starter myself in the first place.... since I have fixed plenty of American car starters... This is a Bosch unit... and in the back of my mind I keep seeing the admonition from Mr. Muir in " How to keep your Volks alive for the Complete Idiot" that the VW Bosch starters were very complicated and required too many special tools to work on.. he, the ultimate do it yourself advocate, said " go buy one and let the pros rebuild it"...
But since I had a chance to stand there and watch them take it apart and inspect it... I will be fixing the old one myself...for back up...since it can back up either the 300 nonturbo Welder or my 240....
But with your symptoms... you probably just need to get the solinoid apart.. or put a new one on... usually on a solinoid you either have a broken return spring... or the copper washer in the end has gotten eaten up by arcing.... often all you need to do is clean and file it... or turn it over to the good side...
Give me a day and I will get the insides of mine cleaned and photographed...
In the mean time... if you are going to look into the actual starter you are going to need a hand impact wrench with a correct fitting end on it...