Basically, long story short, your first minor goal will be to use your SSRs and fill up the rest of the slots with assault SRs. Then once that's done, you'll eventually phase out your SSRs of the wrong element for more assault SRs (or SSRs if you get lucky with them).

Only use weapons that you can max break limit - i.e. event SSRs, SRs, or disaster SRs. Anything from the gacha is too unreliable to get max broken (unless you spend thousands of $$$).

Long term goal is to get a full page (10 weapons) of purely assault weapons of a single element. Having a defender weapon or 2 in there won't kill you, but by-and-large you want all assault. As the poster above noted, as your overall attack gets higher, having a wrong element weapon, even if it's a SSR and fully leveled (so lvl 125) is inferior to an assault SR.

Also noted above, that 'tipping point' where it becomes more or less efficient can be found using Sanahtlig's calculator.