New to Slab Building

After about 45 years since I did my last pottery I'm getting back into it as a combination for my other hobby which is tropical fish keeping.

Well the things I'm working on building are caves for fish as well as decorative pots for aquatic plants. I"m also considering building Castles and other decorative pieces. Slab building I thought would be best approach to this with the second best option being using molds.

I'm basically using low fire Terra Cotta Clay since my source for firing only fires up to a cone 04. I had watched multiple videos on slab building however my joints are never as strong as what I would expect them. Usually I end up with hair line or larger cracks in the drying process along the joints.

I'm looking for some clues as to how to eliminate these. Could it be the high shrinkage of the clay I'm using in the 12% range? Could i be using too wet of a slit made from my clay and water?

I had heard of adding viniger to slip to help the binding has anyone tried this and if so does it help? I have also heard about using paper clay as the slip for a stronger bond? Could I add some fine sand as a grog to reduce the shrinkage to help this problem? Or am I just trying to do something with a low fire clay that should be done with mid range fired clays?