How to dig a trench for a cable for a garden office
You got a bad roll of the dice. Very good ideas and attempts that would work in more normal soil conditions. The problem is, if you continue then you will likely undermine the soil because that small rock will fall into the hole from the sides. That will be very difficult to back fill properly and over time it will settle and form a slight ditch / depression.

At this point I think I would call it on the attempt to minimize the work and / effect on the yard and just dig a proper ditch. It could still be narrow-ish, the width of a shovel. This will give you the space you need to backfill in layers compacting as you go and in the end (years from now) it will pay off in spades.