The major flaw in the alphabet shifting method was that it can easily be deciphered using frequency analysis
A way that you could alter this code in order to make it better, you could not use words with common letters in them although this may be a tad impractical. you could also use multiple symbols to represent the most common letters so that it would appear that you are writing with more than 26 letters. Other than that my other suggestions are to make the code you’re using completely different because quite frankly it is banal and rudimentary.
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