A few general comments:
1. The correct timing to attack after the body splash is to press the button for the next attack during the body splash's hitstop. It's pretty silly since you're inputting a grounded normal while you're still in the air, but this appears to be how it works--inputting the grounded normal after you've landed won't combo. Judging by how the game calculates framedata, I'd wager that the -8 only applies if you don't "chain" into the grounded normal during the body splash's hitstop. If you input a 1 during that hitstop, it indeed appears to be gapless/safe as Rey mentioned.
2. You can Super cancel from d1. It's a pretty tough confirm, but doable. This also means the following works in the corner: 1, 2, 3, 1, 2, 3, d1, Super for 53%, which is the most damaging combo I could find for Doomsday without using interactables.
3. A strange property that seems to be on most moves (perhaps by design of the game engine?) is the ability to cancel the last few frames of their recovery animation into anything. Movement, normals, specials, whatever. That said, here are some BnBs and flashy combos. Hopefully they'll ignite some ideas:
Midscreen
jd3, 1, 2, 3, Venom. 27%.
ES MB, d2, b3, j3, 2, 3, Venom. 29%.
jd3, 3, ES MB, b3, j3, Air Snatch. 41%, 1 bar. You can add a d2 after the ES MB to make it slightly easier, but it will lower the total damage to 38%.
jd3, 3, ES MB, 3, bbMB, j3, 2, 3, Venom. 44%, 3 bars. Good corner carry.
jd3, 3, bbMB, ff, b just as opponent flies over you, 2, 2, 1+3. 34%, 2 bars. Kinda useless, but flashy, lol.
jd3, 3, ES MB, 3, bbMB, j3, Air Snatch MB. 46%, 4 bars.
jd3, f2, 3, Super. 47%, 4 bars. Highest midscreen I could find.
Corner
jd3, 1, 2, 3, 1, 2, 3, d1, Supernova. 40%. Note: the d1 is a pretty tight link, if you're missing it input the second 1,2,3 string earlier.
jd3, 1, 2, 3, 3, ES MB, 2, 3, d1, Venom. 36%, 1 bar.
jd3, 1, 2, 3, 3, bbFB, Transition. The damage will slightly vary depending on the stage transition. ~45%, 2 bars.
jd3, 1, 2, 3, 3, ES MB, 3, bbMB (or ffMB), Transition. Flashy variant of the above. Note: even though the move you want to perform is bbMB, you may have to input it as ffMB since they're directly above you're head, causing input switching to act kinda screwy. ~48%, 3 bars.
jd3, 1, 2, 3, 1, 2, 3, d1, Super. 53%, 4 bars.
jd3, 1, 2, 3, 3, Super. 49%, 4 bars. Easier variant of the above.
4. Baiting mashing/reversals/stupidity in the corner with MB b3 for a cheap transition sounds pretty neat, I'll have to remember to try it.