lxc info gets you some of the data for CPU, memory and disk (depending on the storage backend). Depending on what you’re doing, you could also get the same data directly from the API.
Note that extracting all that information is pretty costly which is why LXD only does it as it’s queried rather than attempt to keep historical values.
As for the migration downtime, the best way to figure that out is to ping your container during the migration as many things can impact the actual migration time. First there is the actual downtime during the last transfer of the filesystem and the container state, then there is the time needed to restore the tasks on the target, then there are network delays as switches learn where the MAC address of the container now is.