I’m thinking the most reliable route might be a train: Venice - Villach - Bled, but I’m open to suggestions.
Use Rome2Rio to see options, as well as a google query. You might also research the Man in Seat 61.
Note that the railway stations (Jezero and Lesce) are not all that close to downtown Bled.
I would consider going to Ljubljana and then bus to Bled. The train and bus stations in
downtown Ljubljana are co-located.