Proper tire pressure?

I have the iScooter F7 that uses 16x3in tires. It says on the tire and inner tube that it has a max psi of 35psi, but I'm seeing online that you generally do not want to inflate to the max pressure, but I'm not sure what I should aim for.

I primarily ride on the street, and while it's usually a smooth surface, there's often cracks in the road or debris like sand or gravel in the path where I need to ride. I'm most concerned with traction and I've often felt like the scooter keeps trying to slide to the side while I'm riding. So far, I haven't actually fully lost traction, but it's unnerving to be riding along at like 10mph and feel like the scooter is fishtailing or feeling like it wants to tip over.

I'm a fairly heavy rider, essentially near the weight limit advertised on the scooter.

What would you say is a good tire pressure to aim for, and is there anything else I should look for that might cause this feeling of slipping to the side while riding?