You could do some overpaints to apply some aspects from your references to your specific object and setting a target to work towards. Doesn't need to look pretty, just to define what goes where storytelling wise and to play with the distribution of shapes (for example main chassis is not just one uniform color but has…
My opinion is that you should probably do away with your 1-textureset-only goal. At this level, it's more important to have clean bakes and UVs, rather than stressing about optimisation, and all the extra problems that mirroring and stacking islands brings. My suspicion is that your texel density is not high enough, and…