I think you’ll really struggle to find a service provider willing to take
responsibility for some combination of an ASA + USG. As far as I’m aware,
you’re going to have to run that in some sort of double-nat configuration
which is going to wreck havoc with any peer to peer applications, voice
over IP, VPNs, etc.
I also can’t say the ASA platform is really a very capable UTM at this
point in time, and most of the features will be extremely limited when all
it sees is 1 IP (the USG). In order to make this work in any reasonable way
you’d have to run both products in a very unsupported way - which again, no
service provider is going to want to touch.
If all you’re looking for is better DNS/DHCP/RADIUS, there are several
products (including Synology) out there that can handle that role in a much
more supportable way assuming you want to keep the ASA in play.