The line you are planning on connecting to is a drain...once a drain, it cannot become a vent. None of your new fixtures are vented. You got away with it when there was only a toilet there, but you can't add a sink and shower or tub in there without running a vent line properly. You have to run the vent line high enough in the house until that drain line becomes a vent, then you can tap into it rather than making a new penetration out the roof (unless that's easier).
Note that dry fitting everything like that will ultimately end up causing you to recut lots of pipe or have to relocate everything since dry, the pipe won't fully seat in the tapered socket of the fittings - you'll lose maybe 1/4" or so at each socket - maybe more.
Also, can't tell the elevation, but the pipe needs at least 1/4" per foot slope all the way - it doesn't look like there's enough, but the picture can be misleading.