I have the same issue. I'm really quite frustrated that my s24 ultra that is supposed to get 2400 nits of peak brightness gets the same brightness as my Galaxy Tab S9 that is only capable of a maximum of 1,000 nits Peak brightness. This is after a factory reset, mainly because I forgot my pin on my phone but I didn't really that's a whole another story, and then accidentally forced a factory reset on my tablet first.
OK, so you've disabled the automatic brightness settings, but you haven't said that you manually attempted to turn up the brightness. My S24U is easily viewed in direct sunlight.
