If you are really interested in a minimal curb, I would look into a collapsible shower curb (they are also known as collapsible shower dams) before I gave up and put in a curb. Google this and study it and see what user reviews you can find. We have not used one yet so I can't give you first hand experience.

The trench drains are great but are much larger than a standard drain and it may not be possible to retrofit them into a second floor shower.

I live in Europe so things are done differently here, but for shower renovations, if I cannot dig into the "chappe" I either create a curb or a platform for the whole shower base. Never higher than 5cm (but no lower as you don't want to trip in).

I researched the collapsible shower dam and am not that enamoured with the design. See for yourself.

Linear drains are meant to look cool. But a circular drain allows more flexibility for tiling options and even a better drain slope.

All the best for this project

You don't have to modify the joists to put in a curbless shower. You can use the level entry shower system from http://www.vimproducts.com/ This product installs directly on top of the joists. Here's one of the showers I have installed using it.