I have a similar stoma, and I am also 2 weeks out from a fresh j-pouch surgery (step 2 of 3). They put me on Imodium and Metamucil, which thickened the output, which in turn decreased both the frequency and the acidity (I mean the enzymes) of the effluent. I can go with only one bag change in 24 hours now, before it was every 12 hrs. I believe this will improve even more as the swelling around the stoma will get less, so the concavity will get less.
I think since the underside of the stoma is more fragile due to gravity, you should use a convex to address the problem there. I had success with convex wafers that are only slightly larger than the stoma. Larger convex wafers tended to hurt me, and also did not work as well; they were not convex around the stoma, but further away from the stoma.
Coloplast sensura mio convex has deep and light convexity, and light convex might work better in your case. Hollister convex was softer than sensura mio, which was not enough to press down the concavity of the underside. The skin and the wafer barely touch sometimes with Hollister convex under the stoma. Even with a ring.
Finally, I am constantly using a tight belt. My loop stoma has the opening facing the underside, not to the front, so the effluent directly shoots into the concavity you mentioned. Unless I wear a tight belt, I cannot close the gap between the skin and the water tight enough to keep the effluent out of there.