LLigetfa
DIYer, not in the trades
I only speed read this thread so in case nobody mentioned it, the Tee-off should be placed after the union, not between the tank Tee and the union. The union is there for easy servicing of the tank and tank Tee. In your pic, you call it a 1" to 3/4" adapter so it needs to be a 1" union on both sides so reduce down to 3/4" after the Tee-off.