Hi Felix,
I’ve had the same issue as you with regards to cut through lines sporadically across my models, see thread with screenshots here: Anybody know why this keeps happening to my models?
I have still not managed to solve it and I don’t fully understand how you fixed it. Could you explain in a bit more detail, it would be much appreciated!
Regarding texturing of your model, I think you need to tweak with the different unwrapping and texture settings, as you can definitely get a higher quality texture than you currently have with 55 photos. I don’t think the issue is not having enough photos. Let us know how you get on either way!