The actual game is beatable with only Rs, when u have grinded all the events-weapons properly.
Also the newer Rs and SRs are getting stronger. For example R kami konohana-sukara provides 20% B-Frame debuff, which can be substitution for amaterasu, as long as you don't have the SSR.
This game is a grind game. SSRs are nice to have but not must have. With time you will get jewels and tickets for the SSRs. And 1 or 2 miracle tickets are affordable, if you want to complete your teams with the SSRs of your choice.