Circuit breakers have to be used with 2 poles in series and mechanically linked to switch together. Check the datasheet for details. Mains fuses with a suitable DC rating on their datasheet are readily available, though.
E.g. bridge less totem-pole designs where rectifying and the active power factor correction stage are fused, would also work unless the firmware gets confused by the lack of input polarity flipping.
And yes, that topology can also go into a flyback transformer or pretty much any other unipolar magnetic of an SMPS.
What it's augmenting is a pretty cheap AIMS power brand modified sine-wave inverter/charger (meh, should have gone pure sine, it causes some of the power supplies to make funny noises) hooked up to a Litime 2kWh LFP battery (very happy with the big battery). TODO on the experiment is swapping out the inverter part with direct 12V conversion for some of the things on the inverter, but I wanted to test the regulator approach with a non-critical component for a month or two first before hooking it to something that can deliver 100A, obviously with a fuse. :-)
There are IP camera 12V UPSes you can get that are ok -- I have a few TalentCell 98Wh units -- but they don't actually deliver regulated power, they just tap directly off of the li-ion cells. Those are about $65.
With an inverter UPS, $50 gets you a unit with a single 12V / 8ah or so lead-acid cell; 90 or so watt-hours _could_ be reasonable except that the inverter draws upwards of 10-15W while running, so these units actually won't run even a router for more than a couple of hours. Also, lead-acid dies after about 3 years, and then you're in for another $15 or and a bit of a pain replacing it. I'm really sick of doing surgery on old UPSes at this point and trying to move entirely to LFP for the 10 year lifespan alone.
If you want really long runtime, you basically have to DIY, but it's fortunately quite straightforward. Larger LFP batteries are about $250 per kWh, sometimes less. DIY'ing it, you can pick your capacity/runtime independent of your wattage, vs pre-packaged UPSes that frequently couple the two.
(I have a big pile of APC UPSes of various sorts, some with external expansion battery packs, and it's quite a hassle to get truly extended runtime from any of them -- I don't want to have a generator, I want 20 hours of extended runtime. I realize I'm a little weird, but I like to hope I'm just ahead of the curve a little on switching. :-)