Scaled floor plan (even hand drawn) will help us help you.

IMO the layout is part of the problem and we need a to scale floor plan to really help.How much money do you have to spend. Start with the lighting get some good fixtures with LEDs in 4000K also the pot lights cajnged to the same LEDs that will at least modernize the lighting. I dislikrejacuzzi type tubs and it looks like you have a small walkin closet . I would wait until you can redo the layout, do you need a tub , would you like a bigger shower I think a complete gut and redo would be a huge improvement and also good ROI when you go to sell.

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.

I would deepen the shower where it stands. Use the little bump-out in the tub space by building a 4 ft high wall even with the regular wall and then be able to use that new wall depth to put in a ledge (in lieu of a niche) at the 4 ft height and a toe-rest for shaving indented into the new wall at the proper toe-ret height. To get storage, get a trough sink with two faucets, leave plenty of counter space on either end, and use 2 ft of your enormous vanity to build a linen closet. If you think you need more places to put things down while getting ready, put a ledge just above the faucets. I will go look for a photo to show you what I mean. Holy cow! The iPad app now lets me make links, just like the regular computer app always has. When did that happen?