Tiles or panels is completely up to you, but after a full gut I would use tile 100%. In LA a condo would in my opinion call for tile were it to be sold so there is that. I would certainly think you need to pull permits and I would want to in part just to 100% sure the shower is built properly, and even then, trust but verify.....................

You need a better contractor than whomever built that. YES, you need permits and inspections. That lack is probably why what you have is leaking. https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer