When it comes to guns/mundane technology in fantasy, I’ve always seen it as a risky but worthwhile long-term investment. A musket is mostly inferior to a bow, and certainly to magic. A machine gun or a tank is another story.
It really comes down to if someone with enough vision and resources comes around and manages to start an industrial revolution without being killed by a mage-supremacist. Even if a trained mage can take on an entire squad of soldiers, in the time the WW PC has been at Tar’citadel so far, hundreds of men armed with machine guns and bolt or semi rifles could be trained and deployed.
As soon as you hit WW1 tech, magic becomes an outdated method of waging war, useful only for support at best. Magic or no, crossing minefields to storm trenches while under artillery bombardment is still going to result in horrendous casualties. The amount of mages you’d need to make a difference is just unfeasable.
Say you have mages shielding the troops from artillery. Leaving aside the fact that most magic barriers in fiction can be broken by repeated sword strikes, what if they wander into a minefield? And what about the actual attack? All of that danger was from just walking towards the enemy.
Then you’ve got chemical weaponry. And all of this is just WW1 level technology!
Anyway, rant aside, glad to hear things are going well. A stealth mission sounds fun, after all, one of my characters is Rhean.