Bigram = phoneme theory (language agnostic)

[ReneZ]

Which glyphs have the highest and lowest effects on entropy? That is, how would we normalise the entropy value of the text to natural language written in the plain with the fewest possible changes?

That is a very good question. From what I have seen, this cannot be confined to just a few glyphs or characters.
The ones that have the largest impact are the ones that create the most restrictive combinations.
And this seems to apply to almost all of them.

Jim Reeds did an experiment along these lines of thought in the 1990's.
One could take the most frequent bigram, and consider that this bigram is really meant to be a single character that has been written out with two symbols.
One could then replace them all, and repeat the process for the next most frequent pair.
One could do this repeatedly in the hope to 'normalise' the entire distribution of pairs.

While I don't think he showed the details, he did say that it did not lead anywhere.

If I may just refer once more to You are not allowed to view links. Register or Login to view. , then Figure 10 shows (I think) quite clearly how fundamental the difference is. Every little square is a bigram. For the plain text on the left, there are large areas where almost all combinations seem possible, while for the Voynich text on the right (FSG transcription) combinations are quite restricted all over the area.