I would follow what
@DoctorHouse has said but will add this,
generally with 5-AIR's slow and steady wins the race, which means keeping everything really nice and stable , you mention you have swapped brands 5 times in 3 years and also tried reducing your dose as well during this period which is pretty much the exact opposite of this, as if your hormones are sensitive to change each time you do these things you maybe inducing an unnecessary shedding phase time and time again.
What I would do from now on is just pick whichever brand you felt was working best or can afford and just stick with it rigidly for the next 1-2 years at least then revaluate your situation.
By the way, I think adding some topical spironolactone in the regime, isn't going to hurt me right? It can only benefit me i guess
( FYI the finasteride in Proscar and Propecia used to be made by the exact same api supplier so is theoretically the same thing at a lower cost, I cant vouch for that 100% anymore though as I haven't used it for sometime)