Thanks! The trim laying on the floor is unfinished. We had originally planned to stain trim to match the floors and the rest of the basement, but then decided to wait until we had decided on a vanity before we chose a trim color. The walls are honestly not as dark and beige as the pic shows. The room is dark as we have not yet installed any vanity lighting. The bathroom will not be a kids bathroom per se, just a basement bathroom, but small children will be using it when downstairs

Hard to imagine how a walk-in shower "opens up" space when it takes at least as much footprint as the tub, but maybe I'm not understanding the plan. A house needs to have a tub for resale. But it doesn't need one in every bathroom. If taking it out helps your options, go for it! Just remember that 9-year-olds aren't going to reliably squeegee glass or whatever, so keep the upkeep parameters reasonable. Also, if you have a clever way to change the j-and-j into two smaller bathrooms, most people prefer that for many reasons.

Your deflection rating will need to be assessed. Deflection ratings are CHANGED (increased) by adding PLYWOOD. Cement backer board is NOT structural. It is always advisable to use TWO layers of subfloor. The first layer is attached to the JOISTS (the slats you see are the first layer and are considered 'useless' with tile so you might as well get rid of them). The second layer is then screwed to the FIRST layer (do NOT 'screw and glue'). The purpose for TWO layers of subfloor = the top layer is EASILY removed if something changes/goes wrong (glue causes issues with removal = damage to bottom layer = more problems then it is worth). Right. So now you know you need TWO layers of plywood. You can get two layers in there with a total of 1.5" (3/4" + 3/4") worth of 'thickness'. Now you have the RIGHT deflection rating (super important) and you have added quite a bit of height. Now you assess how much extra height you need. A well installed porcelain tile can sit (roughly) 3/4" high. You can add cement board to make up the little bit of difference. There you have it. Two layers of plywood + cement backer board to make up the difference.

Yes you need a professional plumber to fix the mess-that little accordion-looking pipe is definitely a handyman special. You mentioned the shower was extended-I’m almost afraid to ask...did the same handyman extend the shower?