Cleon
Legend
Let's let it regrow in 1 day.
How about AC = 10+1/2 natural, as you'd have for swallowed whole? That's AC 20. That's probably ok at this CR. And maybe 50 total hp, as wood is supposed to have 10 per inch of thickness.
Well, the original roots had 1d8 hit points, AC 5, and 3d4 needed to be severed to free a victim. That's an average of 33.75 hp.
So, the AC 20 is OK by me, but I'm leaning towards 60 hit points.
1 day is quite quick, but it seems pretty reasonable considering the entire (above-ground) tree can regrow from its roots "in days or even weeks".
That argues for it regrowing secondary roots in a few hours. (3-12 hours?)