. I can't seem to encourage thicker growth. I fear I may have over pruned them.
100% spot on. To get the trunks to get thicker, you need growth.
Best to get these outside for summer, let them just run and grow.
Keep in mind that if you have plants indoors, sun is very weak. Moving them outdoors in full sun will get them to burn their leaves, so place them in shade for the first weeks, slowly exposing to more sun.