The Maigic "Sixes"..
Had a few of these over the years, Speed, Security, etc. and while they are superb D/A revolvers, there was certainly never a factory option that offered an adjustable trigger or mainspring. Such an opition would go against the intent of the design in the first place- which was to provide a durable, strong revolver that would work no matter what, and be easy to work on in the rare event there was a problem that required repair.
That being said, Rugers have never been famous for great triggers, and folks have been pretty creative in their efforts to improve on them. It seems like I do recall an adjustable-strut mainspring assembly being offered at one time, but the one I remember was for the Single Six & Blackhawks for a short time. I never tried one myself, so I cannot comment on who made them, how well they were made, or whether they were an effective accessory.
Take care-