First of all: incredible game, the last time I had this much fun with a video game was in 2018 with Kingmaker. Best of luck to the entire team!
I really think the Decisive Strike should be tweaked: all the other features of the Battle Domain are passives, so the only domain option you have to actively influence the encounter is your Channel Divinity. However, a single-target attack that targets a very strong save like CON is doomed to fail a lot, and that makes it completely unreliable in combat. I believe changing the save from CON to WIS is a great place to start, and I definitely don't think that would make the feature overpowered.
Thank you for your attention, Myzzrym I love you.
CON is a rough save and relegates the attack to caster-types (assume lower CON). Not useless, jut a very narrow, unreliable use case. Solution, CON, but with a -4 to save?