Every Merge a Nuke player eventually meets the same wall: a Tier 30 nuke is required to trigger Rebirth 5, and that nuke is worth exactly 2^29 base units, or 536,870,912.
Without Auto Merge, manual drag-and-drop throughput is the real bottleneck. Most free-to-play accounts need 8-12 hours of focused merging to bridge the gap.
Rebirth multipliers stack multiplicatively with premium multipliers, which is why Auto Merge + x2 Spawn Speed is the recommended starter loadout.
Cash payouts from city raids are an order of magnitude larger than passive accumulation. Apex raids can pay out 1.2 Quintillion per strike at maxed Rebirth.