Is there a way, short of combining collocation groups, to deal with this
problem?

some options
1 Reduce the number of migration processes
2 Increase the time between migrations eg rather than daily do weekly.
3 do reclamation after migration.

AFAIK the combination of migration processes and collocation groups is
what gives you this behavior. if these two are not negotiable then I'd
recommend option 3 above.

someone else might have a trickier way.

Grant