I have been reading your posts.
I am a surveyor and licensed drone pilot in South Africa and I do a great deal of aerial surveys for various clients, mainly mines. I place Ground Control Points with a RTK GPS before each flight and then processing the data in Agisoft. I get very good results in XY and Z. I have also found tilting in the model, but only if the ground control is not correctly spaced. Long thin areas are prone to this tilting. My work around is to fly at least 4 lines along the line and place the ground control as far as possible on either side with a few in the middle. Then process the data in chunks (with an overlap between the chunks ie, have common ground control points on adjacent chunks) in order to keep reasonably rectangular shape.