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All the Regis need a good recovery option, Regigigas to stall out Slow Start and the others because they have the right stats to be decent walls. Right now, Regigigas only has Rest and Drain Punch to work with.
I think they should have a recovery move unique to the Regis which looks like:
>5PP / Recovers half HP and removes negative status.
This frees up Regigigas' moveslots for post-Slow Start action.
Also, Slow Start would still be a major detriment even if it only lasted one turn, since your first turn would still be a free switch-in/set-up turn against your opponent, so if we're talking about making it viable while keeping the flavor, I support it being shortened to only being in effect the turn after switching in.
With that said, I don't think there's anything wrong with Regigigas as-is. It's nice that the lower tiers have some variety to them, from the mons with bad stats to mons with incredible stats but bad abilities. I like that Regigigas is usable in the bottom tiers because that improves the diversity of the metagame.