A 30A circuit does not push 30 amps. It pushes 230 volts, and the appliance draws however much it needs. As such, no need to return any extra amps... they just won't be taken.
The circuit breaker is a gatekeeper that prevents anything from drawing significantly more than 30 amps (or whatever).
This isn't possible. Stop. Read the details.
A 20 amp mini-split that calls for #12 wire... that can't be right.
Your mini-split has a specification or a nameplate amp rating. You really need to go look at that. I suspect it will be more like 12-16 amps, given that they are asking for a 20A circuit.
We take that number (say: 15.2A) and we derate that by 125%, because that's the law. 15.2 x 125% = 19A. That decides the circuit size we must run.
For 19A, the next larger circuit size is 20A, so we must run a 20A circuit. OK.
With those numbers nailed down, we can compute.
Using the voltage drop calculator properly
First and foremost, for amps, we use 15.2 amps because that is our actual load. Voltage drop is based on our Actual Load, not some number on a breaker trip handle. If your circuit were actually running 20A, you'd be in trouble because of the 125% thing.
Suppose you are buying silver. Someone comes in with 15.2 ounces. But their container says "20 ounce max". Do you pay them for 15.2 ounces or 20 ounces? LOL Amazing how money clears things right up.
Second make sure you entered 240V in the voltage, and not 120V. Not 220 either. US power is 240V.
If it asks for parallels, say 1. Always.
For percentage drop, enter 5.2% because the calculator is dumb, and won't tell you about "close enough" numbers like 5.1%.
Now, when you calculate with proper values, I am 100% sure that it will recommend the standard #12 wire, unless your distance exceeds 180 feet.
Neutral is not needed, unless you go "subpanel"
Let's be clear about this: Code absolutely requires a disconnect switch near the mini-split. The cheapest disconnect switch is a sort of subpanel. It can be a bigger subpanel that might support other stuff too. As long as it has a breaker/switch for the mini-split, and is within sight distance and within 15 feet of the unit.
120V circuits are affected twice as much by voltage drop as 240V circuits, so big wire to a subpanel is also a great way to power 120V circuits in that local area. It sure beats a long >100’ run of #12 or #14 copper clear back to the panel, both in performance and cost.
All that argues for upgrading the mini-split disconnect switch to a proper subpanel capable of supporting other circuits as well. Breaker spaces are cheap, aluminum wire is cheap.
For big wires, Use aluminum.
The neat thing about subpanels and most disconnects, is their terminals are rated for aluminum wire. Your main panel's breakers are rated for that too, which means the long run can use inexpensive aluminum wire instead of pricey copper. This is perfectly safe; the industry learned a lot about aluminum. You simply need to follow the rules: use larger wire sizes, only put it on lugs made for aluminum, torque the nuts to spec, use the goop, and use the new AA-8000 alloy.
You could use aluminum wire as small as #10 for a 20A feeder. However, the small sizes did have problems and that still spooks some people, so I prefer to be up in a bit larger size, as larger wires have always been reliable. And the stuff is so inexpensive it doesn't hurt to oversize. For instance #6 Al is the smallest size I'd use, just because it works better with lugs.
For the price of the #10/3copper you were about to use, you can use fat #2 aluminum, which is 90 amps.