@TaranTheWanderer
Okay, I am literally thinking this through as I am typing this out.
First, I thought about topple, and that it might slow things down, when I made the character. If there was
any way I could avoid it, I would have. But ... there just wasn't. Seeing it play out has confirmed that yes, it is annoying it PbP like I feared it might be. And I appreciate that you're putting thought into this.
Next thought- I am perfectly willing to substitute topple for a different weapon mastery. Up to you. I am not even going to look - although, um, nick probably wouldn't work. Off the top of my head, sap (like a mace) might make sense. Personally, I'd want graze because it's easy to use in PbP and easy to narrate, but that might not make sense.
As for the options-
Don't think that AC+5 works, because topple is about con saves; most monsters have high cons even if their ac isn't that great. Especially at our levels.
If we use the attack roll against their passive con score, it works against the monster because attacks that hit are biased to high rolls.
....so, if we continue to use topple, I would recommend posting the con score. Every time I hit, if you don't mind, I will also use coyotecode roller to roll the monster's save and post it in the OOC part of the attack and determine if it is knocked prone. I am a little uncomfortable with that only because there may be other issues going on that might cause it not to go prone that you know about (or at your discretion), but it should work 90%+ of the time.
Anyway, those are my off-the-cuff thoughts.