maybe try your luck looking for it on myreadingmanga under their pairing tag
could it be this one? But idk where to read it online, sorry. https://otakurepublic.com/product/product_page_3975840.html?srsltid=AfmBOoqMw3eKjQb4FFC6eN91OVM_5wDZchBf-0tkGc63upzuBQRr3x5h
Unfortunately, it is not. I tried looking it like you suggested at myreadingmanga and other sites I could get a hold of, but I couldn't find it no matter what. It has this very depressing artstyle which makes it distinguishable and recognizable. Maybe it's destined that I only read it once and never again for my sake lol. Thanks for your effort though, I appreciate it!
Uh does anyone know a doujinshi oneshot manga between Levi and Erwin where they were doctors (probably), in a cabin or field (not so sure), and Erwin left in the end??? I don't remember the story plot clearly, however there was a syringe involved thingy like and the smeggs I can say was superb.