I believe there might be a hidden message in the decrypted text itself. The reason I think this is that there seem to be filler words added to otherwise ordinary language. Example "I hope you are having fun in trying to catch me". Nobody would use the word "in" at that location in the sentence. If you ask me, something slightly more advanced still remains to be uncovered.
> Nobody would use the word "in" at that location in the sentence.
Except that was, once upon a time, a very common phrasing. As the writer was coming from the 60/70s, you need to adjust your expectations of English.
"in trying to" is actually incredibly common for what might be called "Oxford English". You'll still find it in heavy use in newspapers.
So rather than suggesting the speaker is making a deliberate mistake, or is not a native speaker, it might simply be suggesting a formal education - something that already matches the profile of the Zodiac killer.
(prior to Dave solving this, I was also taking a crack at trying to solve this puzzle https://twitter.com/BlockJon/status/1292555925564395521)