Trap-to-vent distance is determined by how close the horizontal drain connection is to the vent connection in relation to the trap. Where the vertical riser is has no bearing on the 8 ft. limitation. For example; The 3"x3"x2" wye going off to the shower looks like it will have a trap to vent distance of about 18"-24". The vent starts at the connection of the wye. Additionally, the toilet is vented through the soil stack. Even with an offset on the drain before the connection to the stack, this is allowable by most codes for toilets only as long as another toilet isn't connected upstream....which there isn't.
If you did add a vent for the toilet, 2" is code. I understand that it would be difficult with larger fittings to get it done, but because the toilet is stack venting it's a moot point anyways. An additional 1 1/2" vent on a DIY project couldn't do any harm as long as it isn't looped over with 90s as shown in your picture. A horizontal vent must always continue with slope/fall as cacher_chick mentioned and then go vertical to allow condensation to drain or wastewater in the event of a clog.