There are several jstests testing the balancer behavior that could be unified/deduplicated/removed . It would be useful to review them before implementing auto-merge because that change would anyway require to adapt/rewrite most of them.

Having a look at the js files touched under SERVER-65816 could be a good starting point to understand which ones need to be reviewed.

Additionally, keep in mind that in most tests all the chunks share the same size as necessary condition for being backwards compatible with the legacy balancer policy. But once all NOMAS feature flags will be enabled, this will not be strictly needed.