Well, element advantage is really more relevant in the first of the two decision points.

The first decision point is:
Focus on developing 1 primary/1 secondary team (should be the quickest to reach its ceiling)
OR
Rotate elements to match the event

If you pick the former, then rainbow isn't even a thing to think about.

If you pick the latter, the next decision to make is:
Work on only SSR weapons; work with the rainbow grid as you collect more SSRs over the longterm
OR
You work on SR weapons too to get closer to monogrids for each element while waiting for SSRs

The latter here, of course, takes a lot more investment/time.
The interesting question here though, is assuming lower levels of investment (particularly, skill level-wise). Because of how the exp curve and the skill point curve look, and how the gains from weapon levels and skill levels are linear, what you gain versus how much you invest is frontloaded.
While maxing out SSR weapons is the endgoal regardless of how you get there, how you go about in the early stages can be interesting. Instead of going straight into your SSR weapons, you could instead focus on the frontloaded gains and spread resources around on SR weapons to a degree. That pushes back your lategame, but I think that you'll be quicker to reach the power level floor where you can consistently get the missable stuff from events. (namely, ult farming in advents) Once there, you can then decide on whether to continue to spread the resources around, or head into pure SSR developing.