For slow-flying ultralights, how would a delta wing compare to a biplane layout of equal wing area?
One major issue with a delta wing was discovered in the early days of supersonic fighters: delta wings don't handle well at low speeds.  At high angle of attack, they (like any highly swept planform) exhibit strong "Dutch roll" in which roll and yaw are coupled in an oscillation, and because they stall at very high angles of attack the Dutch roll comes on when control authority is at its lowest.

Your plan, to fly just above the stall at very light wing loading, will require control surfaces that make up a very large percentage of the total wing area, effectively compromising lifting area.  You'll also need a very large vertical fin to maintain stability, and much/most of that will need to be movable for yaw control.

Generally, the main reason Rogallo-derived designs have maintained dominance in hang gliders is because they overcome most of these failings of rigid deltas, and can be made lighter, store more compactly, and cost less to construct as well.