Deploying a SaaS solution which is custom for every customer
I'd investigate what would it take to bundle all these customizations into something like a per-customer configurable theme that can be served by a single, more generic application code.

It probably won't be easy and it may take a long time to reach such capability, but it can be done gradually: start with the N*5 repos and, as you inch forward toward this ideal solution you'll see the deltas in the application code from the repos starting to shrink - the differences will gradually migrate towards these custom themes/artifacts. 

When there are no more app code differences left then you can replace the N*5 repos with just 5 app code repos for now common app code and N*5 theme configurations/artifacts repos. Much easier to maintain.