So you've had a plumber come look at the shower drain? Because a sink drain and shower drain are two different things and are not normally constructed or vented the same way. You might have to turn the shower drain into a hub drain of sorts depending on the code in your area.
I like the floorplan and how you worked around the column. Though going down to a 1/2 bath will decrease your home's value. Something to think about if you're planning to sell in the future. A shallow closet would be nice to have but it seems like a pretty small space based on your picture. An open alcove with cubbies is another idea that won't waste space with a door or block the landing like a door would. It seems like you have some space to add more shelving to the utility room but that might be the scale.
Do you have to raise the floor for that kind of toilet? Can you buy a different style that will allow the floor to be the same height? We have low ceilings in our basement and then the ceiling was dropped over our basement shower and it's very cave like. Being a tall person I hate it. If you do have to drop both the ceiling and raise the floor make sure you add a lot more lighting. Actually apply that to the whole space. The right lighting can make or break a basement remodel. Adequate recessed cans and task or accent lighting over the bar will make the space not feel like a basement.
when you say "paint cinderblock walls" what kind of product are you talking? Do your research about what you are looking for and what kind of paint or sealer would be good for that.