In combat with Reavers Row, is there a round of priority after the Reavers Row trigger resolves (where the defending player chooses whether to move a unit) but before combat damage is assigned?

Ruling: Yes, there is absolutely a round of priority between when the Reavers Row trigger resolves and when combat damage is assigned. Combat damage assignment doesn't begin until all players stop playing actions and reactions (pass priority and focus without playing any cards).

Sequence:

• Attack is declared with one unit into a battlefield with one unit
• Reavers Row trigger goes on the stack
• Both players pass priority
• The trigger resolves and the defending player decides whether to move their unit
• Players now have another opportunity to play actions (starting with the attacker) or reactions
• Once both players pass priority without playing anything, combat damage is assigned

Nuances:

• On the initial Reavers Row trigger chain, players can only play Reactions
• After the trigger resolves, both players can play Actions or Reactions on the empty chain
• The attacker has priority first after the trigger resolves
• "When I attack" and "When I defend" triggers create the first of potentially many chains that can be created in a showdown