I put the unions right up to the pump for the closed loop, so it is easy to take off line and clean / service the pump.
I also put unions on all outlets from the four way. (including right under the inlet to teh drum.
Also, if you open up the four way to change the drum, will the water run out of the returns, and if so, during startup will it generate air bubbles again?
I put shutoff valves up close to teh tank bulkheads, thinking it might be better if I was able to isolate the tank there (minimize the mount of pipe that could leak or if it did, ability to cap it close to the tank..
And then the drain line, ... I think in your case, I would have opted for spa flex, so you could move, or adjust the drainline into the splash areaS),, hindsight is great . Aint it??