That's at least some positive news about the garlic mustard. Although 20-30 years is a long time for you and me, from a landscape perspective it's not so long.
Wow. I didn't realize how much of a problem that Wild Parsnip (Pastinaca sativa) is in your region. It looks like you are in the heart of the invasive problem. The more folks can get on top of the problem early, the easier everyone has it. Poor milkweed and Monarch butterflies. They just aren't getting a break at all, between invasives and changes in farming.
Your Wild Parsnip seems almost as bad as the Giant Hogweed (Heracleum mantegazzianum). Here in Oregon, if folks see that one, teams are sent out to get rid of it immediately. The blistering leaves scars for years. What a contrast to all the plants that are so useful! These 2 make Garlic Mustard seem downright pleasant!