Ask siri to set alarm silently?
Solution 1:
Settings > Siri & Search > Voice Feedback
Your choices are :-
Always On
Control with Ring Switch
Hands-Free Only
So really to prevent her answering, you'd need the 2nd option & the ring switch off.
The info in the pref itself says this will only work if you don't use Hey Siri - you will have to use the double-click to wake her.
The alternative is to turn the ringer right down, Siri volume is tied to that. I haven't tested how that will affect an alarm though. It might be somewhat self-defeating.