Can't believe someone above questioned the economics of this. Its a hobby. Its interesting. Its chemistry, engines, and oil. Some people paint or construct scale models, with no economic gain, and this is in that category.
I'd go for at least 15,000 miles given you add a quart about every 2,000 miles, that is pretty often, and it restores a lot of TBN, VII, etc. And you're using a full synthetic. Use a Fram Ultra oil filter to go that long. Or at least use M1 Ext Perf oil filters if not Ultra.
I'm surprised viscosity kv100 wasn't higher, like around 11 or 12 since you put in a very thick liquid (15 oz STP HM Oil Treatment is very thick). You must have some fuel dilution from old rings (blowby), and of course may be shearing down the VII chemicals in the Mobil1 some.
You're smart for using at least some High Mileage oil to help keep the seals flexible and soft enough to perform and not leak in the future. STP has some in it too. I wouldn't put in STP since you're topping off so much.
Saturns seem to have the ability to run forever. Seen this before.