Thanks Middenway, I appreciate you attempting to answer what isn't an easy question.
I think the safest route would be to wait until Plague of Frogs is fully out, and then move on from there. If I'm having to wait years to finish something off in omnibus form, I'll probably just cut my losses and buy them in trades. They tend to be a bit easier to read anyway as they don't weigh half a tonne.
By the time Exorcism is out in trade, it'll be known if it's "safe to read" without most previous stuff, I assume, so I think I can wait. Just want a bit more Ashley Strode in my life