Hello under,
A constant challenge. In your case the PPC plugs won’t work on anything newish anyway since Rosetta went away. As Fredo saliently said, gradually keeping pace is the only way to keep up. The concept of purchasing an audio device that “just works” and you keep for a number of years almost doesn’t exist. With the inability to downgrade new Apple computers to former OSs it becomes even more painful. When the new Mac trashcan was announced one of our colleagues purchased a dozen top of the line old MacPros just because.
You’ll never get the same results as your old plugs, you’ll just have to approximate, you may find something better.
Good luck,
Hugh