Will winding a single DC cable on a spool cause any issues?
Loops with a bit over 1 turn are similar to straight cable as far as current handling is concerned. 18AWG 2 core cable is rated for ~7 A, well above your maximum current, so heating should not be a problem.
Resistance is ~21 mΩ per meter, so the extra voltage drop will be ~ 0.5 A * 0.021 Ω * 0.3 m * 2 conductors = ~6 mV, which is insignificant compared to 5 V.
Inductance of the loops is about 0.2 μH, which will probably be insignificant in a cable carrying DC power.