What is best substrate for long lasting outdoor mural painted similarly to fancy painting on cars?

Aluminum would give you both light weight and weather resistance, but paint has a hard time sticking to aluminum. There are a few ways to prepare aluminum so that paint will adhere well.

The classic way is to thoroughly clean the surface to remove any traces of oils (power washing can do this), then spray with a phosphoric activated vinyl wash primer. This is an environmentally nasty product, though.

Epoxy primers work well (first a two-part epoxy polyamide, then a one-part epoxy ester).

Or you can try to get anodized aluminum, which takes paint well.

All of the prep could be done by a metal finishing job shop, if this sounds like too much trouble.