Does this help ?
https://www.opticscentral.com.au/blo...s-with-a-dslr/
I soley use Newtonians for both visual and imaging and found moving the primary mirror inwards up to 10 or 12mm has resolved most of my focus issues. All depends on where your focal point sits.
A 2 x Barlow could be your ultimate solution
Cheers
Martin