I think I have this tribag too. I was planning on trying it out, wrong way round in front of the steerer.
https://www.topeak.com/global/en/products/top-tube-bags/277-tribag
That's exactly what I use. (Well, mine's the earlier model with the mesh top, which I've augmented with a couple of Ortlieb repair patches to keep the rain out.) Works nicely.
I got a custom frame bag made for my df bike last year and it worked out very well for adding a very useful load space. It wasn't that expensive, being cheaper for a custom bag than buying off the peg from the main bikepacking brands. The success of that makes me wonder if there's a place you could fill out with a custom bag on the streetmachine. There's obviously no triangular frame suitable for filling but is there any space centrally between the lowrider panniers or somewhere else?
I reckon the space between the lowriders is ripe for exploiting, but you need to allow room for the lower chain tube to move up and down unimpeded as you change gear. It'll get spray from the front wheel, of course.
Do you load stuff onto the bladder? I'm planning four panniers plus rack top bag or loading. A bladder strapped onto that then a weight on top, well that's a potential fail in the bladder I reckon.
Sure, I often strap a rack bag on top of it for longer day rides (while touring I tend to keep the top of the rear rack free except for last-minute shopping and drying wet clothing). The bladder tends to squish down through the gaps in the top of the rack, so doesn't end up under much pressure. It's not going to burst anyway, the bite valve will piss all the water away first.
I've only had an Ortlieb water bag fail once, by the material becoming porous at one end. I attribute this to regular washing out with Milton-alike, which presumably pooled at the lower edge and became concentrated enough to damage the material. I've been more careful with its replacement, rinsing out with plain water and air drying, and not had that problem. It doesn't seem to have gone manky, either.
I've seen YACF's loudest 5 year old jump on one repeatedly (water-filled, with the regular cap) without bursting it, so they're pretty sturdy. It's sharp objects that are going to be their nemesis.
I'm looking at stands too. There's a lowrider mounted one and a chainstay mounted one that's included in HPV's list of options. I've heard with loaded bikes and recumbents the rear mounted one is more stable. Anyone got any views on this?
I reckon the lowrider stand is more stable, because it's closer to the centre of mass of the bike lengthwise, and further out from the centre line, but I confess I haven't tried a Streetmachine with a stand on the chainstay. I've used stands in both positions on DFs, and the Streetmachine lowrider stand is much more stable than either.
A leftfield option might be a custom length click-stand hooked on the rear rack just behind the seat perhaps. They're a sturdy but light pole with a hook on top. You use it hooked onto the bike frame somewhere and the bike leans against it. The steering of the df is usually held straight with something too. Overall it's supposed to be very stable even on uneven ground. I know from my son's frog bike that the b stand doesn't work even unloaded if the ground isn't flat.
More faff than the lowrider stand, but yes, that should be even more stable.
With USS, a Streetmachine's front wheel has about 45 degrees of travel before the bar hits the seat - this means that you don't get the situation you do with uprights on stands where the front wheel turns 90 degrees, rotates, and the whole bike falls over.