I have a plus bike (a lovely Bombtrack Beyond+) and it has all the bosses you could possibly want, and will take 27.5×3″ tyres and 29″x2.25 tyres, but it is pretty expensive at £600 for a frame in the sale, so although I love mine, it doesn’t tick all your boxes.
A few comments that may help hopefully.
Mine came rigid as a plus bike, and it was fine, but its a lot more fun with suspension (even the cheap Suntours that I have). You can pretty easily smash through the 2″ of air in the tyre and it jars pretty hard when you do. Also steel rigid forks can weigh as much as suspension forks, so weight difference can be pretty minimal. I do still put in the rigid forks every now and again (they are on it now), but its a bit of a faff to swap, and its just not as much fun.
I also swap in and out the 29″ and 27.5″ wheels, and again, although it has the 29″ on at the moment, I much prefer the semi fat ride. Its a bit lower and more forgiving, but they weigh loads more (my 50mm rims are 750g each so the rims alone weigh as much as a posh set of 29″ wheels) so can be a bit of a slog. I use the 29″ wheels for long XC rides as I just don’t feel like I am in control in the woods.
My bombtrack geometry is probably “old fashioned”, but it was designed more as an adventure bike and as above you need to decide what your main use will be and then if you can accept the limitations when you come to use it elsewhere. I use it mostly for messing about in the woods at 27.5″ and I did the SDW on it last year on 29″ wheels to try and save some weight and rolling resistance.
I would say at £700 the Onza is great value though