Early game, Ariados/Spinarak is really underwhelming.. but mid-game is really when he shines the most. Most of the time, you'd rely on Night Shade more than any of his other moves to deal significant damage -- and you'd have to keep him as your highest level just to make Night Shade quite effective during that period.
But one thing you're probably neglecting/forgetting is his Egg moves. He could acquire Poison Jab as early as level 37 from Beedrill and Signal Beam from Venomoth(37)/Volbeat(25). Either way, those moves will make your Spinarak's start better and more reliable compared to how he would be from early-game. However, this requires patience and time as you'd have to wait until your other bugs are good enough (and of course, reach the daycare center). For me, that wouldn't be much of a problem... since keeping a party of 2-4 other bugs is ideal until mid-game for most of the time. But different trainers have different styles, so idk how that'd work for you >_<
Another option is to relearn Bug Bite. However, this is pretty late already, as the move relearner is found at Pastoria (D/P/Pt). But it's a much better choice than say, Signal Beam.
Toxic isn't bad either, and definitely won't be wasted. The move doesn't rely on either SpA or Atk anyway, and you can always make sure that he'd be dealing significant damage against any opponent this way. Toxic-Protect is a pretty popular combo too, although commonly used. With his puny speed, it will benefit him the most.
That said, you'll definitely need investment to really get Ariados start good. But that wouldn't be too long, because as mentioned, you could do that as early as mid-game starts.
IMO, an ideal moveset i'd go with would be:
Gotta admire your bug monotype challenge. Spread the love for bugs!
