What kind of conduit should I use with UF-B to reduce depth of burial?
I'd use conduit with THHNs in it and save the UF for wiring the office-shed
One thing most people don't realize about UF is that it can be used as a substitute for NM with the same makeup; the NEC permits this in NEC 340.10 point 4, and while it's usually uneconomical, it's a good way to use up spare lengths of UF.  This is because while it's legal to use UF in conduit, it's a waste of conduit fill compared to using individual wires, and also much harder to pull through the conduit compared to individual THHN conductors.
I'd use a 1.5&quot; and a 1&quot; PVC conduit at 18&quot; to top of conduit (20-24&quot; deep trench) in your situation; if you don't need anything else, you can use the 1&quot; conduit for 3 (1 circuit) or 4 (1 MWBC = 2 circuits) 12AWG THHNs to a 60A non-fusible AC disconnect box on the outside of the office-shed.  You can then run your branch circuit(s) from that box, and just provision a 15A feeder breaker at the panel you're running this from, with the 1.5&quot; conduit left as a spare for future provisions of more power, or fiber Ethernet to the shed for that matter.
If trenching that is out of reach, you could use rigid or intermediate metallic conduit in the same sizes in an 8-9&quot; deep trench, but keep in mind that that conduit is going to pose an obstacle to landscapers and others going digging in the yard in a way a 24&quot; deep PVC conduit won't.  You'll also need to use metal (RMC) expansion fittings in the riser at each end instead of the much cheaper PVC versions, in addition to the conduit being considerably costlier than PVC, and occasionally requiring either compression (threadless) couplings or a threading machine.
So, it's either more work trenching, or more work hauling, cutting, and threading conduit, with the latter creating a hidden annoyance for landscapers as well.  Pick your poison.