Now there is a need of connecting all branches to AWS so it is good opportunity for me to consider switching to BGP instead of creating new multiple areas, ospf instances etc.
do you mean you were thinking about having *sd-wan like* bgp over the cloud, to overcome hq session and hardware failure?
well, I think the most obvious thing is bandwidth traversing the cloud, and perhaps some security concerns as well. 900 spokes.
but, if you insist that cloud *route reflector* method - i was thinking about dmvpn.
just a thought