Your nebula shots are fantastic!
Are you using any filters?
I use a Baader Moon & Skyglow filter, which works well, but it's not narrow band.
I think my focal length on the M51 shot is about 1540mm.
I'm using an 8" SCT (FL of 2032mm) with a 0.63 focal recucer/flattener, but the reducer is quite a bit closer to the focal plane than recommended. Based on my fov, it looks more like a 0.76 reduction, which leads to the 1540mm FL.
Mounts: EXOS II PMC-Eight, LXD75
Cameras: ZWO ASI294MCPro, ASI385