If you insulate with 2" of closed cell foam between the studs, with only 0.5" between the stud edge and the you'll have R1.5 framing short-circuiting the R12-R13 foam reducing it to about an ~R8 wall as a "whole wall-R", after the thermal bridging of the framing is factored in. With the 2.5" you could put in R8 econobatts and end up at about R14-R15 whole-wall, but there is a slight risk of wintertime moisture accumulation at exterior stud edge on the above grade portion since it'll be running cool.
Since the studs aren't structural and only need to hold up the gypsum, not the house- either install them turned 90 degrees or use 2x2s (finger-jointed stock if the wall flatness matters much to you), and get the full 2" of foam between wood & concrete. Then use unfaced R8 econobatts or split R13 unfaced sound-deading batts to fill in the 1.5-1.75" deep cavity between the gypsum and foam. At that point you will be well north of R15 for whole-wall R, about 2x the performance of what you've proposed for only a modest (if any) increase in cost. Any fiber insulation needs to be a compression fit- you may have to split R19s if it's deeper than 2" from gypsum to foam anywhere, but you can compress R19s by quite a bit. (At 3.5" a full R19 batt is the same density as an R13 batt, and performs like one.)
Instead of sill gasket under the studwall plate, use 1/2-1" XPS (pink/blue/green sold as insulating sheathing). Sill gasket is pretty marginal as a capillary break, and worthless as a thermal break. Even R3 is enough to keep the wood well above the temp of the slab year round, minimizing it's moisture accumulation, and you can use standard grades rather than pressure-treated. Use TapCons through-screwed to the slab to keep the stud plate from migrating out on you over time.
The efflorescence is almost always a bulk-water issue from exterior drainage (assuming the footing is above the water table), but it could also be wicking up from the sub-soil from trickle-springs or high water table. With 2" of cc foam on the walls there will still be some water vapor making it through to the basement (which is fine), and as long as you don't use batts with facers, poly sheeting, or something highly vapor retardent like vinyl or foil wallpapers to trap moisture in the studwall. If there is at least a foot of above grade exposure on the exterior the concrete will then dry primarily toward the exterior, and you may see minor efflorescence or spalling on the exterior after some number of years, but if that happens a lime-mortar parge on those sections would be sufficient to protect the concrete from moisture wicking damage for the next century or so.
Green board won't be necessary in your basement & climate. In places with higher summertime humidity cool basements can accumulate moisture just from outdoor air infiltration- 70F dew-point air leaking into a 68F basement is a mold-factory! But your summertime outdoor air dew-points are in the mid-40s F according to Weatherspark.com datasets, so as long as you don't have ground moisture issues or you're putting up a winters-worth of firewood in there the basement humidity will stay below 60%RH, maybe even below 50%.