Forest is forest, no matter what species. Pretty much all trees follow similar principles so to make any forest look natural:
Trees always follow light. Branches that are shaded usually get weak and eventually die so, to make your group look natural: Prune so that branches grow out and away from other trees in the forest. Prune so smaller trees grow out and away from nearby larger trees. Interior trunks may only have a few low branches, most will be in the canopy above the outer trunks. This may vary depending how close your trunks are planted. Use your imagination - if that tree grows there, how much sun/shade will it get because of the bigger trees nearby?
Make rear trees smaller and front trees larger and thicker to 'force' viewer perspective. Small trees at the back of your forest will make it appear they are further away and increase the viewer perception that the forest appears bigger than it is.
Similar principle with outside trees. Bigger, taller tree somewhere just off centre acts as a focal point to attract the viewer's attention. Smaller, shorter trees to each side gives the impression the forest is bigger.
Prune trunks for taper where possible. Replace thick upper trunks with side branches if you can get them to grow upward and look natural as a trunk. Good taper gives the impression of older trees. Also enhances perspective and makes the tree seem bigger because our brains expect upper parts of a big tree to look thinner. Looks like you have plenty of possible upright branches to cut back to in the 2 larger trunks. You'll need to look for similar possibilities in the others.
Don't let trees get too tall. We expect older trees to be thicker. Excessive height makes trunks look skinny. Height-width ratio.
Also a tip from experience: Keep outer trees in check by pruning more. They have more root run compared to inner trees. Also more access to light so they can grow faster and thicken quicker than inner trees. It's important to maintain thinner trunks toward the outer edges for perspective but it's amazing how quickly some outer trees will thicken because they have light, root space and, often, more branches.