To be honest your post is not clear, for instance:
eg : I have for client A : 20 urls/landing pages
You mean URLs and not separate domain?
If URLs what is wrong with the 20 landing pages? Are they all the same product?
we are considering developing and managing a portfolio of brands to replace our current domains.
You are considering replacing how many with how few? 200 to say 20?
What is wrong with using some of those names for your branding purposes?
I have been down the route of hundreds of domains, 4-500 at one point IIRC, and I'm now down to 50 and will probably release another 15 this year.
At one time, up until about 8-10 years ago you could have many sites and funnel them, it's almost pointless these days, in fact for me, even 301'ing exact match domains drives very little traffic, I lie, it drives none!
These days your efforts are better spent on creating one super site ensuring all your SEO work is well done, unique text, unique images and no blackhat stuff.
By all means 301 your existing domains at the new one(s) but choose your new name carefully and develop your template with view to big upgrades as you add in all those existing domains to the master site.
Doing it this way will also rank brand new pages rank much faster and rank better.