Livre d'Or

Yeah, it's acceptable to say no to things even if you feel like You're The Only One Who Could Possibly Do It (which I doubt is true? and even so?) - "It's the High Holy Days, I'm the only service leader in my community so I have a disproportionate amount of work to do, I haven't been part of that community for a decade and it's an extremely complicated and sensitive subject and given my other obligations I simply won't be able to do it appropriately even though in other circs I could" is one of the most acceptable reasons to politely decline something I can think of!

You say "this feels like a hugely scary brief to be handed" but it is a choice to accept the brief or not! And if you do choose to, that's also a totally valid decision but it just seems in this post that you're thinking of it as an assignment you've received, rather than a major project you're deciding whether or not to take on. It might be worth revisiting the fact that you do have agency about whether or not you do this.