Two additional things:
1) There is a third way to initiate a visceral attack, normally restricted to large bosses or boss-like enemies. Against enemies with breakable parts (always the ones too large to parry or backstab), one or two of these parts will put the enemy into a staggered state and make them vulnerable to viscerals for a few seconds. Usually this is the head, but some exceptions like both of Amelia's hind legs or the church giant's exposed shinbones do exist.
2) Beasthood will also boost the damage of visceral attacks when it's active, as it does with all physical damage. A meter with 300+ Beasthood will increase visceral attacks by a large margin (as I show in this video
), and I've seen a video where someone managed a 5-digit visceral attack on the Moon Presence. If Beasthood's 70% boost fully applies to visceral attacks, then your 8856AR maximum could be pushed up to 15,055AR with >300 Beasthood in the meter. I also wonder if serration/holy damage applies to viscerals? I haven't tested for that, unfortunately.
I also think it's worth mentioning that visceral attacks don't use the weapons own base damage or scaling modifiers, since that's a very common misconception. This would also explain why scaling gems didn't seem to have any effect on visceral damage.
Side note, I knew visceral attacks counted as physical damage, but had no idea they were thrust damage specifically. Snazzy.