You might also look at Raphael's drawings. His studies are remarkable. But you won't see him on video actually execute the line. There are some really excellent images online of his preliminary studies, the Royal Collection Trust has some images that are really well imaged - https://www.rct.uk/. (Raphael's study of his "Massacre of the Innocents" demonstrates why he is a master) I really appreciate that you did this research to answer your own question and shared it. Thank you for that.
Nice job. The composition shows some really good decisions. The choice to make the background quite realistic affects the decision not to be "realistic" vs "what looks good" on the lion. That having been said, drawing lions from imagination is really commendable. I think some of the moves below may bring the contrasts into a cohesive whole. For me, I think that a little more time on the anatomy of the hindquarter, leg and the face of the lion would bring this to life. Aaron Blaise may offer some good reference in that effort. The dark shading in the middle of the body sort of bifurcates the lion's form, it cuts the animal in half, one might look at how that shadow could ease/lighten as it proceeds vertically around the body. remember that body is cylindrical/rounding while layering back, even in the shadows, look at how your own body looks as you curve to one side, it layers almost like a stack of coins or one ball resting in front of another. The rear of the lion may need more contrast with the immediate background, the wall is perhaps too similar in value to the lion's hind end. Well textured architectural background; make sure the perspective is consistent; the trees beyond in the archway and above the frontal masonry ruin may be too abstract and light, using as much detail there as you have in the masonry could help to push the lion and architectural ruin forward in the image. If you alter the rear archway to make it a niche instead of an opening, that could do a few things for the image, however, that may not work symbolically, if there is something you are trying to say with the opening. What a vaulted (half dome) niche would do visually is provide an opportunity to shade above the lion's head with darker tone to push the head volume forward in space. if you cut a little black construction paper in that shape you can test it first prior to drawing it . Again, darker trees in back above would also place the front arch in space more clearly. As it is, the open light zone above is difficult to discern whether it is a wall material change or recedes to something else. For me the 6B pencil/graphite stick is a really useful tool. I hope you find this perspective constructive and useful. Best,
Nice job. Focusing on your background question, to my eye, the background tone is a little dark. The tone works well for the dog in the front, but the tone seems too close to the middle dog and could possibly lighten for the dog in the back. Possibly a gradient that works from center, or working with a gradient that reinforces the composition may push the dogs further forward. The scale of the texture in the background seems a little large, scaling that down, so that you see it as a texture and not its constituent parts, might serve to simplify what you are seeing. That scale reduction may have the result of darkening the texture, but you can take it down reducing the opacity or saturation. The middle dog kind of melds with the dog on the right due to their similar colors, to me, this affects the composition, I see reflected light back to the middle dog at the leg, but could that reflection be used higher toward the head to clarify the order in space. The similar colors in that area formulates an isosceles triangle low at the center which to me, creates a bit of confusion within the composition. I see this more evidently in the thumbnail. One might also try lightening the dog to the rear further using a little license. I hope you find these comments useful. Best.