I like your digraph chart maker and encoder.
Some thoughts. I really like digraph in general, not necessarily for the 340. It is fast and easy to encode, you can make some mistakes without wrecking the entire message, and it is slow to decode. It was a very popular military cipher a century ago.
You don't need a digraph chart, obviously, to encode a digraph cipher. But charts can be generated from a keyword, random, or different types of digraph ciphers to help better visualize what the chart or cipher would have a propensity to do. Digraph ciphers can be compared by comparing their charts. My encoder is separate from my chart making spreadsheets for interchangeability.
I also brought up poly literal transposition, something that we briefly discussed a long time ago, and I think that you wrote some computer programs on it. I am trying to think of ways to create messages with period 15 / 19 and / or pivots.
Diagonal transcription of two letters at a time + homophonic with a dominate period 2, simple and possible. Maybe also a dominate period 5, which would become period 29 / 39.
Digraph + poly literal transposition + homophonic, maybe but maybe start simple with the above and consider digraph later. I don't see how digraph could create a dominate period 5, but maybe it is possible. I it though because I could see someone thinking to encode two letters at a time, and then also transpose two symbols at a time. A simple work combination.
Sorry about thinking in two directions at once.
