The most basic cart is filled with distillate mixed with terpenes. If you use quality distillate (maybe from one of the verified facilities) and good terpenes (preferably derived from cannabis), mix well under light heat (perhaps on a hotplate with a stir bar), you can make some decently nice carts for yourself.
If you aren’t able to obtain distillate you can lightly process your shatter to make it more suitable for use in a cart. Decarbing is a good idea, just do it properly like @cyclopath said. If you are running into viscosity issues you could try dilluting it with distillate of an alternative cannabinoid, or maybe even winterize it?