The other problem is that there were LOTS of reform movements in the Church. Most of the big monastic orders were founded as 'pure' reactions to the 'corrupt' existing orders. Then, of course, most of the new guys got popular, got lots of donations, ended up rich - and became as corrupt as the existing houses.
To keep reform happening, you have to have a long term force keeping the hierarchy's eye on reform (not on power, money, etc.).
To some extent, that happened with the Reformation - the Protestant churches posed a threat that the RCs met with the Counter Reformation, which eliminated a lot of abuses that had existed up til then, and let them stay abolished.
Hmmm... What if Orthodoxy and Catholicism overlapped in more places. If some western ruler converted to Orthodoxy, and much of Central Europe had both kinds of churches. Then you might possibly be able to get the competition you'd need.
Oooo... Just had another thought.
If the bardic/skaldic immunity spread further, and secondarily, some took up the habit of excoriating the powerful (nobles, rich merchants, churchmen), you might, just MIGHT, be able to keep reforms in place.