There are no bad sets, this is a purely preference mon and he is god tier.
252 defense bold is a great set in its own right and I personally love it. The types of teams that I build incorporate pivoting, fearless switching, and usually very delayed hazard removal (because good spinners still give very unfortunate free turns to the opponent). I don't always have a perfectly healthy slowbro to bring in late game, because in the midgame he's been abusing regenerator and fishing for burns.
Therefore I am not using calm mind to be able to survive hits against magnezone/rotom in the late game, I'm using it to pressure panic the opponent and get them to switch their own pokemons around to rack up hazards and chip damage. Usually a midgame single calm-mind to get a feeling for what answers they have (who is running toxic? who has electric/grass/dark coverage? who has taunt?) followed by either a scald fishing or just switching out and getting the regenerator. Only later, if I actually need it at that point, will I get the real boosting started.
You will miss those 252 defense EVs against strong uturns, +2 talonflames, thunderpunch metagross, life orb mamoswine, burnt conkeldurs, LO weaviles. When post regenerator your mega slowbro only comes in at 50% and takes hazards. It could be the difference between just LOLing at physical attackers and healing up, or dropping dead. Pre-game you have no idea what % you'll actually be when you come in against their physical sweeper for the 5th time. Every defense helps.
There are many ways to play with him and he his far and away my favorite mega. To each their own: fire blast is amazing, ice beam is amazing, rest vs slack off is an endless debate. But this is the set I use and it has been awfully successful, I highly recommend it. Don't theorymon so much about getting past greninjas, just use a porygon2 or a noivern or something and play smart.
Slowbro @ Slowbronite
Regenerator -> Shell Armor
248 HP / 252 Defense / 8 Speed
Bold Nature
IVs: 0 Atk
-Scald
-Psyshock
-Calm Mind
-Slack Off
Naturally I paired it with a cleric (sylveon) and something that pwns magnezone (rotom-h) and something that pwns rotom-w (celebi) and from there it was just managing hazards and dealing with the meta's threats (so I used porygon2 and tentacruel)
tl:dr 252 Def Bold is really fucking good