No, you're not overthinking. You want to 'get it right' and the absolute truth is, auto grats cover it all but, and there's often a but isn't there, if you want to add extra it is entirely between you and any staff member you wish to 'reward'.
Don't listen to any onboard chat [if you do hear any] or any comments about how much will be given extra; It is personal and up to each individual.
May I suggest though, if you do want to give a small token, use a major currency. Roubles and the Manet for instance are a tad too country specific🙂