I’d be tempted to build it like a floor with joists, noggins and hangers…and build it right across from wall to wall especially if you still have headroom to walk on it. The money you save on the uprights and extra bolts will probably cover the increased size, just the OSB sheet or whatever you use on top that will be a bit pricier.
I’ve build a wall to wall 3m x 1m platform but in my case the bikes (and washing machine) reside underneath slotted into a pallet style wheel rack that is vertical against the wall. And all the junk/wheels/tyres go on the platform.
I roll my bikes under and the rear wheel slots into the pallet…they are snug but not tangled, and I reckon five bikes is a comfortable number under a 2m platform…if you put them on top it might be a bit more unwieldy. Consider all the way across and trailer and bikes all underneath.