Instead of the usual 2:X pairings for merge/SYC, group the elder kingdoms together for a large-scale Stake Your Claim event and let them choose where they go.
By now, the players in 1590, 1637, 1638, and 1639 all know the usual process for SYC and have friends and enemies in each of the elder kingdoms, many of whom may have been separated unintentionally in previous life cycle events.
Due to population concerns, I would recommend at least 4:5 or 4:6 to ease server load and caravan/creature scarcity in these kingdoms.