I uh... CB jolly mega or SD does a lot to many offensive teams and now with a good speed tier it now nukes most offensive pokemon with mega kick
252+ Atk Huge Power Diggersby Mega Kick vs. 0 HP / 0 Def Garchomp: 264-312 (73.9 - 87.3%) -- guaranteed 2HKO after Leftovers recovery
-1 252+ Atk Huge Power Diggersby Mega Kick vs. 252 HP / 0 Def Landorus-Therian: 184-217 (48.1 - 56.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252+ Atk Huge Power Diggersby Mega Kick vs. 252 HP / 252+ Def Slowbro: 171-202 (43.4 - 51.2%) -- 99.6% chance to 2HKO after Stealth Rock
252+ Atk Huge Power Diggersby Mega Kick vs. 0 HP / 4 Def Zapdos: 288-340 (89.7 - 105.9%) -- 37.5% chance to OHKO
252+ Atk Huge Power Diggersby Mega Kick vs. 0 HP / 0 Def Heatran: 120-142 (37.1 - 43.9%) -- 1.2% chance to 2HKO after Stealth Rock
the calcs are nutty against many offensive teams and SD drops bulky teams
To be fair, you posted calcs versus almost defensive stuff sans Slowbro (which always run Heavy Duty Boots, and while Adamant helps it wallbreak, I highly doubt it runs this nature due to sacrificing such a potent speed tier.
But some more reviews.

+ Levitate
I get the desire of Levitate Gengar. We love it, its funny its cool. But my main gripe is that even with Levitate Gengar isn't much. It just isn't competing with better Ghost types in Dragapult and to a much lesser extent Blacephalon. I respect the utility and if this was a weak slate I'd vote for it purely on vibes, but the potential line up is too good for me to put a vote on a nod and a wink vote.

+ 20 Spe
This is a cool concept imo. Diggersby has always been a weird pick thanks to its lack lustre 78 speed tier, making it so that Diggersby is threatened by literally every single offensive team. 98 speed makes Diggersby a real offensive threat. Outpacing the likes of Tapu Lele and Urshifu-Rapid-Strike is huge for Diggersby, turning it into a very legitimate threat. As for having zero defensive counterplay, I think this is heavily exaggerated. Yeah it breaks alot of stuff but It isnt as black and white as one thinks, and many ways to play around this Pokémon exist. (Essentially gitgud scrubs). Really the funniest thing is that Scarf Diggersby is now a legitimate set now with its great speed tier.

+ Body Press and 20 Spe
I subbed this, and everyone has raved about it, so yes the mon is very good.

+ Electric/Dragon (and Clanging Scales) and 20 Spe
This is a cool concept. It keeps all of it's key checks but is a vastly improved upon Pokémon. Trading it's ability to check Fairies with one to be a very good Bulky water type punisher (even more than before) while also just shedding the crippling x4 weakness. I still am wary of its rather pathetic 75 HP / 70 Def / 70 SpD however, as this heavily limits the defensive potential of such a potent typing
252 SpA Tornadus-Therian Hurricane vs. 0 HP / 0 SpD Toxtricity: 107-126 (36.7 - 43.2%) -- guaranteed 3HKO after Stealth Rock
This replay simply means that while Tox beats Torn in a 1v1, as long as Torn hits all of its Hurricanes, Torn will most certainly outlast its supposed check. Even so, I think its a good buff and makes Tox a very credible wallbreaker in OU.

+ Poison Touch
I think the buff is kinda mid, but I see the vision here. Its an objective buff that makes Knock Off and U-turn a bitvh to switvh into, and slimming down switchins makes it easier for mandi to take advantage of its threats via momentum. Yet still, I still believe most of Mandibuzz's most potent flaws still beung aggressively present, despising Knock Off while being a team's usual Dark resist, bein rather easy to wear down due to this and its susceptibility to Toxic and its somewhat easy nature to be taken advantage of due to its passive nature. Its rather unfortunate though as I love Mandibuzz as a Pokemon.
All and all a cool slate, will be voting for

and either

or

, leaning more towards Digger.