It sounds like wall-mounted is not going to be an option at all; anything sturdy enough for safety would have to be mounted into studs. Honestly, I wouldn’t even trust construction adhesive; it could possibly fail, pulling the tile off the wall, and injuring one of y’all in the process.
As far as I know, that leaves this option:
Drive Medical Handicap Grab Bar This style is somewhat infamous at our house; we have never been able to find one that grips well enough that it is safe. However, we do not have porcelain over steel enameled tubs; our tubs have curved walls that slope inside the tub - so while they look very pretty, they stink at holding one of these in place.
There is also this - although again, please proceed with caution:
The Rock Steady Cane I use this when I travel, and the grab bars are on the wrong side of the toilet in my hotel room. It’s not perfect, but I can make it work. I have also been known to take it into the shower when things are on the wrong side - but I only use it there when I have a helper.
This is the last thing I can find:
The Stander Security Pole. It is a tension spring mounted bar; how well it works I have no idea.
I imagine you are already familiar with sliding transfer benches; those are a safer way to get in and out of the tub, and most of them can be turned to store inside the tub when not showering. (I have to use one of those when I visit our daughter’s house).
I hope you figure out a solution that works for you!