Nomad

you can use your very client dongle and it will work like a Nomad 256 dongle. the only thing you can't do with the old hardware version of the dongle is upgrading to a higher output count. so if you use nomad with your dongle as primary or backup, your system will be limited to 256 dmx output addresses. if you use it in a client role, the output count of the dongle is ignored.

If I do use Nomad as my backup, am I able to control output independently of the console as with a Net3 system or would that require a Gadget?

you only need the Gadget if you want a DMX connector or your nomad system. nomad outputs either Net3, sACN or ArtNet. nomad as a backup behaves exactly the same as a console in backup mode would: the backup console behaves like a client (except that it has the additional purpose of taking control over the rig when the primary fails), so you can control the whole system either from the backup console (or backup nomad) and the primary. but you need to be aware that your old client dongle will pull down the output count of your whole system down to 256 output addresses.