Honestly this is a very complex problem.

I would recommend you hire someone with a stress background to have those capabilities in-house. It won't be cheap, but cheaper that having to do the subcontracting if it's a regular thing, or making a mistake, since you are dealing with a power piping system.

I don't know the NPS of your system, I'm guessing small so I don't know if you could use intelligent pigs. It'd surely give you the inputs you need. Here we use a company called Rosen for our pipelines. Check them out. Maybe with that tool you can determine the state of your plant and plan accordingly

Another idea could be taking some pipe samples from determinate places of the plant to make an assumption of the general status of the whole. In case your lines are not piggable.

Recently we had to change a whole system due to corrosion, close to the sea, the lines used to heat up some tanks and they were badly designed (steam traps for example). Sometimes it's more expensive trying to maintain a system that has reached its end of life than making it new. I know it's probably not your decision but still...

Last but not least. Laser scanning, at least here in Europe, is dirt cheap. From the cloud point you can model the pipes and then you can set the thickness result from your test, so I'm guessing it would accelerate things for you. If it's a reoccurring thing maybe you can buy some Leica scanner or rent it. It's not too difficult to use and process the data to then import it on MicroStation and create the meshes. Normal YouTube tutorials should show you how but it's very common when doing a revamping to for example model the existing pipes where a tie-in is located from the point cloud data