There's no such thing as a "service receptacle"
What it means, quite directly, is this:
Most houses have general-use receptacles around the outside of the house, for holiday lights, barbecues, bouncy castles, hedge trimmers and all sorts of things. Make sure one of them is within 25' of the heat pump so the maintainer can plug in their vacuum pump, which has a 25' cord.
That's it. That's the whole of the rule.
If there's already such an outlet within 25' as the cord flies, you already comply. Finished, move on.
Otherwise, expand your network of general-purpose outdoor receptacles so there's one nearby. Putting it right next to the heat pump is pointless and counter-productive. Dedicating the circuit is worse. Why? GFCIs don't last outdoors, and heat pumps don't need vacuum-pump service very often. A GFCI on a dedicated circuit will be stone dead next time it is needed. What good is that?
You want that outlet to be on a circuit you use all the time, with GFCI protection ideally indoors but at least where a GFCI problem will be soon detected. As such, you are better off extending an existing outdoor outlet circuit, or if you don't have one, extend this one to be as generally useful as possible.
I would use a green #12 for ground. I calculated the conduit fill at 26% for these wires in 1/2" EMT.
EMT provides the ground, and there is no need for a ground wire. However if you do fit a ground wire, you must follow the rules. One is that you must land at every junction box - so you must splice to the box and not ignore the box and splice only to onward wires.
To save conduit fill, use a bare copper ground.
is no issue running a black and blue #10 to the condenser, I would use a green #12 for ground
Negative, Ghost Rider. Your 15A circuit requires #14 hots and #14 ground. You are enlarging to #10 hots but only #12 ground.
250.122(B) Increased in Size. Where hots are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type grounds, where installed, shall be increased in size proportionately according to the circular mil area of the hots.
So #10 hots = #10 ground.
Hey, you're the one who wanted a ground wire in EMT :)
Other than that, the plan looks fine.
However, with the "service receptacle circuit" possibly removed, you might only need a simple disconnect instead of a subpanel. Remember too that subpanels have a working space requirement that must be flat and kept 100% clear 100% of the time - 30" wide (not centered), 36" deep and 6'6" tall. Nothing can ever be stored or let to grow there.
I personally am very against putting subpanels in places where people will tend to be inclined to put things. That forces someone to be the "working space nazi" and nobody wants to do that.
Can I use one 1/2" EMT conduit from the panel to an outdoor single gang box with a 20A GFCI, and then transition to the NM whip?
With two circuits in the EMT? Sure. With 15-20A circuits in conduit, there's a simple rule (derived from a stack of rules that KMJ overshared lol): Up to four 15-20A circuits in any size conduit. With 1/2" conduit that's not even a concern, since 5 circuits won't fit lol.
Also, there is no useful use in distinguishing the colors of the 2 hot wires. In fact, it helps identification in a crowded conduit if both hots are the same color. So if you have enough blue, use blue-blue.