Merchant
At the end of the day, it would be bad for Zayn to absorb and annex close to 150,000 soul particles and that was not because of his own well-being. He had to be a bit careful not to attract too much attention.
After creating the first Soulring it was much easier for Zayn to absorb and annex larger amounts of soul particles. It was not a problem for him to absorb 150,000 soul particles since every wave of Gluttony Termites rewarded him with only around 1000 soul particles, to begin with.
Annexing a thousand soul particles at once was not further difficult, no less when there was a soulframe that had to be stabilized and filled, but Zayn chose not to fill his soulframe.