The adjacent third floor bedroom, which is in front and to one side of this lone toilet cubicle would be the best place for an additional full bath. However, the pitch of the existing horizontal stack would raise it above the floor level with the additional length.
Does anyone have experience with the Saniflo macerating pumps that allow bathrooms to be put in basements and remote from stacks? It would only be a few feet to this stack but without something like this the entire stack might need to be changed.

"It would only be a few feet to this stack but without something like this the entire stack might need to be changed."

Per code the pump type systems may only be installed in instances where a gravity flow drain cannot be achieved. It it can be done by changing the existing stack, that is your only option.

In addition, before you begin planning to connect a water closet to that stack you need to know how all the fixtures below are connected. If there are presently any unvented fixture arms connected to that stack you would need to install auxiliary vents on those lines as well, because code prohibits connecting an unvented fixture arm to a stack at any point below the discharge of a water closet.

I looked at the pictures I took and I think that another option would be to used a rear discharge wall hung toilet such as made by Duravit with the Geberit in-wall concealed tank system.

However, the new bathroom, although adjacent to the existing lone toilet would put the new toilet at least, outside this radius. However I would not have a problem venting this as needed. It may actually make sense to run a new stack and eliminate the part of the stack that runs horizontal/diagonal through the stairwell.