Comment by Gormo

You needed this: http://mainbyte.com/ti99/software/s_carts/editor.html

Or this: http://mainbyte.com/ti99/software/s_carts/mini.html

I was in the same boat as you. Games like Microsurgeon were doing crazy things with the TI-99, and nothing that seemed possible with XB matched up. I didn't learn about the assembly programming tools until much later.

The retrocomputing scene for the TI-99 is pretty active, and there are people writing modern games that blow even the commercial software from back in the day out of the water. Check out Realms of Antiquity, along with anything written by Rasmus Moustgaard.