If you want a Pokemon to get around its low speed stat, the obvious answer is to give it priority moves. Alolan Raichu has a number of priority moves, such as Ally Switch, Helping hand, and Fake Out, although fake out is probably unnecessary because you want to get speed swap off as soon as possible (like on the first turn). Ally Switch and Helping Hand will help support your sweeper without having to be fast.
Setting Electric Terrain is useless if you've got the speed of a Pokemon like Aggron, you won't be outspeeding anything anyway (Aggron's speed without investment is 70, which doesn't even outspeed base 80s while Surge Surfer is active, and you shouldn't invest any EVs in speed if you're just going to swap them away), so electric terrain is a waste of a moveslot.
You could definitely use Rising Voltage, but if you opt for this move, you might want to include Tapu Koko on your team so you can have an alternate "electric spam" mode, as Alolan Raichu will also be able to do a lot of damage.
I'd recommend a set like this:
Raichu-Alola @ Focus Sash
Ability: Surge Surfer
EVs: 252 SpA / 4 SpD / 252 Spe
IVs: 0 Atk
- Rising Voltage / Nuzzle / Thunderbolt
- Speed Swap
- Ally Switch
- Helping Hand
If you don't want to use Tapu Koko, Nuzzle and Thunderbolt are good moves too. Focush sash is very important in order to get Speed Swap off, as Raichu is pretty frail.
Finally, in the current ruleset of VGC 2021, you can use one "restricted legendary." Calyrex-Ice would probably work much better than Aggron; it's much stronger and bulkier all-around without any crippling 4x weaknesses, and has much more potential to snowball the game with Chilling Neigh. Additionally, it will help prevent Raichu from being useless for the rest of the Game, as Calyrex can OHKO Landorus-T, which would otherwise give Raichu (and Aggron) a lot of trouble, Helping Hand will have a much bigger impact when used on Calyrex as opposed to Aggron, since it's attack stat is so much larger, and since a fast Calyrex-Ice with multiple attack boosts is very scary, your opponent is likely to focus it down, making ally switch much more devastating.
I hope this helps!