no character could fill Gertrude's shoes and defeat Jonah, preventing the Apocalypse. for one, Gertrude herself was unable to accomplish that. her disappearance/death is the first thing we learn. the reason why Gertrude was able to do her thing and stay hidden from Jonah was because he both respected her and knew that whatever she did would also benefit him (he wanted the rituals stopped back then). it likely took years for that respect to grow and for him to grow complacent. comfortable. that character wouldn't have that. Jonah wouldn't care about them stopping the rituals because he now knows they don't work. because of Gertrude he'd be more careful. he wouldn't let them stay out of his Sight. like when he goes to Jon's office when he can't see who Jon is talking to. that character wouldn't be allowed the same freedoms Gertrude were because Jonah both has the experience and also he has planned his ritual already
in the end of the day it doesn't matter if a character is "like Gertrude". they wouldn't succeed either way, both because the actual Gertrude didn't and because Jonah would be more prepared this time around