good idea indeed. even Roserade can lay some Toxic spike too (roselia can learn it), but i still like Sludge Bomb more. Reliable power, 30% poison, and it can hit Flying type (who immune to spikes) much better. And to breed Sludge bomb for Roselia, follow these steps (i know, too much works, but it's a worthy reward):
Muk------> Stunfisk (you still need it for Gengar) -----> Wooper (too easy to get on Azuril Island)------> Snubull (an event pokemon) ----> Roselia. That's what i use.
And for Venoshock, I got no idea how to breed it to Roselia. Maybe i'll search for it if you want (because i've never used it before)
And yeah, i think this combo Roserade- Venusaur with Sunny is too powerful for Fern or Luna to handle
And for the TM Sunny day, i joined Magma gang (the only way to get Lanturn in-game), so i got it long before Randomus. If you go with Aqua gang, you have to go to 7th street to get it (so it'll be after Randomus)