I went about ten hours into the game before I realized I had something like 30 pounds of old weapon and armor mods in my inventory. I couldn't find a way to reuse them or scrap them for parts so I just sold them all for spare change. Seems like a waste that I can't even reinstall them. Is there something I am missing to make better use of them?

You can actually reinstall them, just go into the mod screen for the appropriate weapon, and instead of "build" it should say "attach mod" or something like that. You can also scrap them by dropping them in a settlement and then pressing "scrap" in the workshop menu.