Existing drain too shallow for new basement bath?

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Eastern Washington
I am getting started on adding a basement bath. I tore up the concrete where I plan to tie into the existing 4 inch ABS main drain; about 3 feet from where it leaves the house. The existing pipe is much shallower than I expected: At this location, which is the farthest downstream where I can tie in, the top of the pipe is 6 inches from the slab surface. The slab itself is two inches thick, with sand in between.

I plan on running 4 inch for the new drain, so in theory I will have enough depth to get the proper slope while keeping the pipe below the slab, but it still seems awfully shallow. Adding to the issue is that I was hoping to install the new basement toilet about 12 feet away from the wye. If I keep the toilet in this location, I will use a street 90 up to the flange, but it seems to be pushing it.

I cant find any specific burial depth requirement from my city for drain depth under the slab in an existing structure when it is inside of the building foot print, so I am assuming that as long as the drain is not embedded in concrete I should be ok.

Should I remove all doubt and alter my plans so that the length of the new drain is reduced? Is it typical for existing drains to be this shallow?
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