How do you properly cook stuffed cannelloni?
I think your cooking method is fine and the problem lies in the stuffing. I see a few things that may be contributing factors to your issue.


Make sure you are using the minimum amount of olive oil you can. You don't want excess in your stuffing.
Spinach tends to release a lot of moisture when cooked. Wilt the spinach and squeeze out as much moisture as you can before adding it to the garlic.
Ricotta tends to have quite a bit of moisture. It can vary by brand. Try draining the ricotta through a strainer for a good while to let the excess moisture drain off.


You might also try reducing the amount of bechamel in the stuffing. If I'm looking at it right you are currently using as much bechamel as cheese. 

And last but not least, a commercial trick I learned is to add a very small amount of bread crumbs or cracker meal. I've seen that done in both baked dishes and salads like tuna, chicken, etc. It's kind of a hack to handle extra moisture.