Thanks for the feedback! The reason the upgrades are only in one direction is to ensure all your tiles are "connected" together. We weren't able to enhance the visibility of that, but it was something we did to make it really fit into the theme of the jam even further than just upgrade paths. We also did want to add more upgrade tiers but time got the better of us and we had to cut scope down quite a bit from our original ambitions!
We are happy to hear you enjoyed the game regardless of these short comings!