Right, that would only work for NM in NC.
Well, if it's 20 small NM cables, and clamps can only handle 2 cables each, then yes, I'm proposing drilling 10 holes in the junction box/enclosure/wireway/whatever, and one big hole to attach the large PVC nipple (< 24"). So if the existing PVC goes through the rimboard, the box needs to be recessed inside a joist bay, could be a bit difficult if the joists run parallel to the rimboard.
Each cable gets clamped to the box and then passes into the panel through the conduit nipple (< 24") to the panel. The PVC nipple is sized so that the conduit fill is no more than 60%, using the major diameter of each cable to calculate the equivalent circular area.
Of course, I also like the idea of one (or several) large screw mount cable ties inside the panel. Seems simple.
Cheers, Wayne