would that give me 20 psi?

No - You only theoretically get about 0.43 psi per foot of water head -
and that is ignoring the wall friction effects of a 5/8 in hose.

Use auto wash soap and warm water & a rag or sponge. A pressure washer
is hell on bearing seals etc and will promote corrosion in the areas
that get pried open from the wash pressure.

A rainy day also makes a good wash day with a soft water rinse.