I speak from experience... but then again.. what do I know... I generally plumb from the bottom.

I'm glad you haven't had to fight a priming problem on your installation.

If your pump is below waterline at rest, then you are right.
If your pump is not below waterline at rest, then the pump will be required to prime itself (which most AC pumps are quiet, nonpriming, vane pumps).

With a thru transom pickup, it will need to be mounted low enough on the transom that it doesn't suck air from the surface (like a drain eddy), but that generally means the pickup will be lower than the pump - so the next issue is making sure that your pump is lower than the waterline as well.

AC pumps don't like to run submerged, so the tendency is to mount them high enough in the bilge that water won't ever get to them - just make sure it's still lower than the waterline.

As mentioned before, all of my installations have been through the bottom of the hull along with the pump being below the water line. That is the best way in my opinion. However, I have had a couple friends that couldn't justify drilling a hole in the bottom of the hull so they mounted it in the transom. Both occasions they mounted the pump below the water line so all was good. Any good marine A/C manufacturer should give detailed instructions detailing the obvious when it comes to this... should...

I`m about a foot above drain plug with the pickup , everything else I`m keeping below the water line, if prime is a problem I`ll do a T after pump.
My only concern is the strainer, I wanna keep it accessible and serviceable so in the up position. The AC manufacturer dude says point it down..
Only time I need ac is at dock. I guess I could also just shut the valve of anytime I`m not running the ac to keep prime..

put it on there, run the boat, then try the A/C see if don't have to bleed the air out..will be airlocked...have a marine a/c business...done many installs....