That looks like output from the rubydebug codec, which uses the amazing print library. I would suggest converting it to JSON and then parsing it with a json filter.
I assume you are consuming that as a single event.
A simple gsub will get you most of the way there
mutate { gsub => [ "message", " => ", ": " ] }
If you have arrays you may need to remove the indexes. If your arrays look like
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