Depends on the closet collar, some have a little bevel under the flange that requires the pipe to be cut back a little below the floor. A std. 4" x 3" female closet collar should fit perfect inside 4" pipe if it's plastic. Don't rely on the piping to hold the collar down, anchor it to the floor...
Don't bet on it being a threaded connection in the wall, can't see on the pics real well but it looks like a swt. joint. Lots of hose bibbs/sillcocks are copper swt. and I.P all in one.
You can get different length drain hoses for dishwashers, I've seen them up to 12'. Pull the dishwasher and put on a longer hose. https://www.amazon.com/Eastman-91218-Universal-Fit-Dishwasher-Discharge/dp/B07RGRHP6D/?tag=atomicindus08-20
If your'e doing a new shower only install some 4 port T+S valves can be installed upside down using the tub spout port for the shower and cap the upside down shower port. I know some of the Kohler valves work this way, the tub port is larger than the shower port and it'll supply more volume to...
You'd be surprised at what you can do with Tinkercad if your'e just starting, very easy to pick up. My home laptop doesn't seem to have the guts to run Fusion 360, tried Blender also and was too slow.
If that floor is going to be 4" thick with a 4X3 offset collar come right out of it with a 3" st.60 or 45 and you'll probably get it to 15" easy, if not run it at 14" and after a while you'll never notice it.
Maybe it's regional thing but most drop in tubs like your's we set with the rolled lip of the tub just proud of the tile and finished off with your caulk of choice. Setting it below the tile leaves you with an area that will trap all sorts of garbage, just my opinion.
Look at a dielectric fitting the rubber washer and plastic sleeve prevent any metal to metal contact between the 3 pieces, thus stopping any current flow through the fitting.