I play Disc Priest and I'm also coming back after years, I'm figuring the same thing out. Here is where I'm at.
Every damage spell is the same macro (from memory, hope it's right, may be a [harm] tag in there too):
#showtooltip
/targettarget spellname
Then I just target the tank, cast PWS then do my damage rotation which all target the tank's target. Refresh PWS every 15 seconds.
If I need to do AOE healing, I cast Power Word: Radiance then continue my damage rotation. I don't cast PWS on everyone individually it's too tedious, so I don't need to track it.
I've solo healed heroics effortlessly and also done raidfinder Argus the Unmaker. Item level 200, I've been playing for under a week.
An alternatve to consider is instead macroing the heal spells the same way and target the enemy you are dpsing, then you PWS whatever the enemy targets.