So I reckon this is how I'd deal with this tray.
The first full tile on the right that's half in the shower tray with the dotted line thro it I'd leave in one piece initially. (The triangle piece after u've enveloped it) Then place it in the tray packed up to the drain just to see how high the right side lifts off the level floor. Am I making sense?
If I thought the level was manageable I'd prob lift the 4-5 tiles around it to meet it, as it's only a few tiles. Then u'd have a slight fall outside of the shower entrance too.
If not I'd cut the 300 piece off and use some kind of profile to make the separation between floor and tray. For no other reason than aesthetic's even just a 2-3mm right angle profile rather than a grout joint. To me it then seems intentional as opposed to looking like a floor pieced together.
It's a shame with a bit more thought on their part it could have looked a lot better.
I'm sure others would approach it differently but I think that'd be my plan.