I bought the game on itch, will I ever have access to the content in the steam version
Not sure I understand the question.
If you're asking if your save will work on the Steam version? Yes, it should carry over if there are no compatibility issues between versions.
If you're asking if you'll get a free Steam key when we launch there for owning it on Itch? No, you will not get a second copy of the game. Both Itch and Steam (along with every other store front) get the same game, and it all gets updated at the same time. We treat each store seriously and no users will feel left out because they own it somewhere else.