Mescalinic
Oh sorry, what I meant to say is that there is no point in running "Withdraw and
Rest" on the same set (I have edited that word now) - use Iron defense instead of Withdraw if you use Rest. I give the reasons for that in the sentences following it. Of course, there is nothing speaking against amnesia, in fact, I highly recommend it.
The other point I wanted to make is not that Calm mind + Scald is always better than Toxic. My point is that if you already have Calm mind + Scald on your set, then there is no further benefit to adding Toxic in a third moveslot. The benefit of using Toxic to break through stalling mons is that you can run Amnesia which allows you to handle certain strong special attackers who 2HKO you when you have only 1 stage of SpD boosts whereas +2 SpD gives you a third turn in which you can then heal up.
Of course the downside of Toxic is that you have to rely on trying to make your Toxic-immune opponents struggle (including all substitute users and Mega-Sableye/Absol). And rest gives you huge trouble. You also give up on the chance of a nice Scald burn.
You can also run a set with Scald + Amnesia (Iron defense, Slack off). You get the benefit of resisting most special attackers very well and you do not have to worry about the plethora of toxic immune mons. This of course weakens your matchup against recovering mons.
As you can see all these choices have their pros and cons. Handling a certain type of enemy better than others but falling short in other areas. It ultimately depends on what situation you expect to face the most or what role your Slowbro needs to fulfill in your team in order to patch weaknesses of the rest of your team composition.
Same goes for your criticism of Rest. It has problems against strong attackers. On the other hand it beats Toxic which other sets lose against.
In the sets your proposed I would make the following changes:
Set 1: If you pick rest, then use Iron defense instead of Withdraw.
Set 2: Pick Iron defense instead of Withdraw. If you can boost up to +6 SpA, you should have no need to PP stall. You also do not have to worry about your opponent using Rest because of your high damage.
Set 3: On a Rest set I would prefer to run Amnesia as with only +1 def boost you will likely not survive until you wake up.