You could look at an alphabetic shorthand such as Orthic:

https://orthic.shorthand.fun/

The learning curve is very gentle, you could learn it in a day. Honestly the hardest part is getting used to reading it fluently.

You can also look into various systems of abbreviations developed for telegraph (Evans basic English code), or you could look into using Yublin, which is basically taking all 2-letter combinations and assigning the most common 676 English words to them. Personally I like the idea of Yublin, with the addition of suffixes to modify common words so the word "add" might be "ad" in Yublin, but to make it "addition" you might turn it into "adn" and to further modify it to "additionally" you could write "adnly". This way you get more words out of your limited number of bigrams instead of polluting it with a word plus all it's commonly used variations. Write that shit in Orthic and you'll be flying.

Food for thought.