Right now there are only 4 runes that deal damage when invoked. 2 are applied to an enemy and 2 are for allies (judging by the base effect). only 3 are avaliable at level 1.
At level 1 you could do 6d6 (avg. 21) across 3 fort saves for the cost of at least 4 actions.
At level 9 when you get the fourth you could do 24d6 (avg. 84) with 3 fort and 1 reflex save and 5 actions. Also the target needs to be grabbing the ally with
At level 18 with Annihilating Composite Invocation, you could do 10d4 + 62d6 (avg. 242) with 4 fort and 1 reflex save and needing 7 actions. You also need to cast an extra divine and occult rune on the target to meet the needs of ACI. You would need to use an etched rune to do this.
At low level the damage does not seem absurd, but with scaling on the level 1 runes all being 2d6+(2d6 per 2 levels) coupled with multiple saves pulling the damage closer to the average, it could be for anything with a week fort save.
The combinations could be a cool to work around this stacking damage. I do wonder how short the list would need to be.
I brought this up earlier in another topic, pretty sure. There was a pathfinder 1e teamwork feat for casters to add special effects by casting two elemental spells together using a held action.
https://aonprd.com/FeatDisplay.aspx?ItemName=Elemental%20Commixture
You had a primary spell and a secondary spell, where the secondary wouldn't have its normal effects, but instead modify the primary spell to have +1 DC, +1 CL vs SR, immunity to standard counterspelling, and a secondary effect based on the two element tags. Temporary blindness and nausea, save vs prone, movement speed reduction, catching fire, staggering and concentration checks to cast spells...
An ability to stack two simple damage spells and, instead of just upping damage, add secondary effects and boost the save DC by a bit, could be a very neat way to keep damage to reasonable levels while also adding a lot more tools to the kit for debuffs.