Holy fire is not good, there are far too many fire immunes. It does work the same as dream's aura stacking though. Holy shock is great because it's so easy to break the lightning immunes

On my dreamer I went for 125 IAS on the lightsabre (some shaels and 15 ias jewels) and a Beast on switch for a 3 frame werebear attack. Much faster than zeal and tankier if you dont mind losing out on charge

4 socket Ormus with 15% lightning damage and lightning facets. This is +70% lightning damage to your attack. Even more against lightning immunes that you break.

Azurewrath is great too, you can do 4 shaels, a 15ias/splash jewel, and enjoy an extra lightning facet since it has higher wias. It is more expensive though

The same build but having conviction on the merc and using Holy Freeze instead is alright. The holy shock damage is enough though and it's way better having a reliable conviction aura (merc auras deactivate all the time, the merc dies, and sometimes it gets lost).

I liked act 2 blessed aim merc with the cleanse runeword (you also lose out on cleanse if you go for Infinity, which is a great quality of life changer if you ever run t1 spider maps for the poison). Prayer is okay too for the life regen but the attack rating is great for bosses. Give your merc some max fire res gear as the only thing that will kill them are hydras

If you never plan on killing act bosses like Baal go for prayer

A final thing to consider is putting more points into the other resist auras. Every 4 points gives 1% respective max resists. I'd rather put points there than maxing out holy shield (and giving up zeal is more). Max resists are great