Two attorneys should be giving you good legal advice.

My advice: document, document, document!

The settlement amount should be based on "real damages". Grown trees are very valuable, take pics of the surrounding trees after they cut through (if you don't already have "before" pics). Loss of wildlife habitat is also damage, especially if you can document your prior hunting and fishing on the property. Ruining a pond is also damage, and quite expensive since construction costs on a replacement pond will be considerable!

They should actually be taking actions right now to stop/minimize erosion. That is doubly in their self interest since they will have to pay you for damages due to erosion, and erosion that exposes the pipeline can cause damages and/or significant liability on their end.

Basically, I would document every kind of damage and lost use you can reasonably support. They will probably make a low-ball offer to your attorney. He should then counter high. When Targa does not accept that, he can send over your exhaustive documentation. They will not want to go to court where your attorney keeps introducing documented additional damages that they have not covered in their settlement offer.

IANAL. However, my wife did an oil & gas section while getting her degree at SMU Law School in addition to her property law training.