No to the tiled bench. Sharp edges in showers are a bad idea. You also can't reach your back or undercarriage to wash them when seated. It makes it impossible for someone to help you wash. A movable shower chair gives you flexibility and they have arms to use for lowering and rising.
If you don't want to put grab rails in straight away, put patressing in the studs so you can retrofit. Don't tile, use Multipanel or similar, it's much easier to clean.
Have your thermostat and control away from the shower heads so you don't get hit by cold water when you turn on.
Remove all sharp edges and wall hang everything, it makes cleaning easier.
If you have a heated towel rail, don't put it near the shower. You don't want to be grabbing it if you feel unsteady exiting the shower.
Maximise free space, you'll need more than you think if you need mobility aids or a carer.
Have a non slip floor surface. Altro Aquarius is good, but not painful underfoot. Underfloor heating is worth considering.
Think about lighting carefully.
You need space around the loo for rails and/or someone to assist. Comfort height as pp suggested, but if you are short, it needs to be tailored to you as they can be too high.
These are just top of my head suggestions based on the remodel we did for my elderly parent. I cared for them including personal care. Showering is a whole other thing when you are less mobile and require help, as is toileting.