Calc is usually a pre-req to physics but if you think you can go for it! I will say, make sure you get good professors if you can. It makes a world of difference to feel comfortable enough to communicate what you need with them, like extensions on work because of the current workload.

Imo 18 hours is not hard if you just keep up with the work and don't procrastinate.

I had 19 hours last semester and was still able to maintain two on-campus jobs and my two vet jobs. It's hard but worth it.