When I visit Culebra, I'd like to make a side trip to St. John and/or St. Thomas. I'd like suggestions on how to do that in the most direct way possible. Lower cost options are the best. Thanks.

Your only viable option is to fly.

There are no "direct" flights from Culebra to St. Thomas so unless you are willing to go by private charter flight you have to first make your way back to SJU.

The most "direct routing" would be to take an Air Flamenco (https://airflamenco.net/Home) flight from Culebra to SJU then connect to either a Seaborne, American, Jet Blue, Cape Air or Air Sunshine to St. Thomas.

If you wish to visit St. John you have to first make your way to St. Thomas (as described above) then take a ferry from either Charlotte Amalie or Red Hook to St. John.