Hi John,
yeah, unfortunately it is often like that - improving one part makes other parts worse. I can be kind of a vicious circle leading to dozens or even hundreds of CPs. Lot of time and effort, but sometimes the only way if you can’t get more images. At least you got the pipe on the top now - I’d be surprised if that part would get much better with the images you have right now.
I am pretty sure that leaving the gravel away will not make things better, since that is only the reconstruction stage, but the key is the alignment.
You could play with the following settings:
Preselector : the max number of points that will be used for matching or creating tie points, it should be 1/4 - 1/2 of Max features [https://support.capturingreality.com/hc/en-us/community/posts/115000782031-Points-count-and-Total-prjections]
Max features per mpx/image : e.g. 20k/80k - more features will be detected (only if enough of the defined quality are present)
If not, you can set Detector sensitivity to high, which will allow for more but also less quality features