Barry, yes and no
For planetary, I bought a colour QHY5L-ii (which is relatively inexpensive), it's basically a guide cam...but I'm pretty pleased with the results I've been getting recently. One thing is that it seems to struggle with noise a bit on the fainter objects...I've not had a Saturn image that I'm particularly happy with. A more sensitive and/or less noisy option is on the wish list, but a simpler solution would be to get a barlow to put me somewhere in between f/10 and f/25.
For DSOs, I bought a Canon 1100D DSLR on closeout. I've only really tinkered so far but motivated by what I've been able to capture.
I'm not convinced there's a sweet spot between the two. I'd say the combo you have now should work pretty well to get started with.
Regarding a mono camera....they are more sensitive as they don't have the bayer filter in the way, but you then need to contend with a filter wheel. For me, that's more complication with all the fun I've had getting the subject on the chip
