I DM more than I play and one of the reasons is I really dislike being in situations where I feel like control of my character is taken away from me for shock value. It’s happened enough as a player to make me push for DMing when we role-play just so I can avoid the negative feelings among friends.
Not everyone’s like me, of course, and if you have a set of players who are into playing the narrative the way it is presented, this sounds like a good way to do it. I like that you have a non-death way out, even if it’s the “bad” way, so that it makes players feel like they had some agency in choosing a heroic death.
If some of your players are like me, I have had heroic deaths be enjoyable as a player and DM (actually one of my favorite ways to go out of a campaign). These have all worked around players knowing it was going to happen. As a DM, if I was going out like this, I’d just tell the players up front: “I want to run a mission where your characters all have heroic deaths. Are you in?” It takes away the shock value, but it adds so much. First up, you have dramatic irony. The players know their characters are going to die, but the characters don’t. Also the player’s don’t know how yet. And because you agreed to it up front, they’ll be looking for and excited about the opportunity. It also allows players to know the level of emotional attachment appropriate for the character in the moments leading up to their death. This may seem obvious as a one shot, but if they didn’t die, they could show up in the core campaign as NPCs, and your players might be excited by that possibility.
I think Matt Colville’s video on killing PCs was good, but I watched it a year ago. Maybe worth checking for advice also!
I like the way your adventure ends because those characters could crop up in the main campaign as villains (if they side with the wizard) or remembered as heroes by local bards (if they side with the shaman). It wasn’t entirely clear reading through it why they should help the shaman, though. Those dudes sound pretty evil. I didn’t get a clear view of who the good guys were. I’d have a harder time sacrificing a character heroically for a morally grey reason.