Unlike typical drawings, there is not a continuous vertical stack from the roof through the basement floor into which branch drains tie. Instead, the 3" stack ties into a 3" horizontal drain in the basement ceiling, downstream from one of the toilets. The drain exits the house fairly high up on a basement wall.

For the remodel, the easiest way to conceal the 3" stack is to have it come down a wall at the upstream end of the horizontal drain, where it would attach via an upturned Y.

Hopefully, my feeble sketch shows the idea.

The toilet and other branch drains would enter via upturned Y-and elbows. The second bath would have its own 3" branch with separate vents.

Is this layout, with the stack vent at the upstream end, acceptable?

Thanks for your help.

You can vent the lav (sink) first, and the tub shower first also, with two vents or even a single vent, but all these vents can be and must be vertical. Consult others too, and do not ever think that any single internet post contains your answer.

The lav vent is fine as shown and if the drain line from the tub is 1-1/2" and equal to or less than 6ft in length is does not require a vent.

If your under the UPC the lav is fine.

If the tub line is 1-1/2" it is limited to 3' 6" from the trap to the main drain without requiring a vent.
The code would also allow you to increase the pipe diameter by one nominal trade size, which would make it 2" aand that would allow you 5ft from the trap to the main drain.