Will there be an incentive to migrate between the Shards to keep number/nodes per shard as balanced as possible?

A shard with a lower number of nodes has the same statistical probability of a transaction crossing into or out of it. The lower the number of nodes in a shard, the higher the probability of being selected to create a temporal proof for a crossing transaction. This higher probability gives you an incentive to service this shard as creating a temporal proof is the way you get rewarded for work on the network.

For more information, see: https://papers.radixdlt.com/incentives/#shards