You brought up an issue that I'm not sure "how it works".
You said "Earnings are low because we withdrew from less aggressive/mostly fixed-income accounts."
I would think that earnings are prorated based on the total balance in your overall 529 plan and not just the earnings in the individual investment (account) that you withdrew from. You show probably verify that with the plan administrator. Normally the 1099-R would answer that question, but you're not getting one. Please let me know what you find out.
Edited 1/6/24: Here's one reference that agrees, with me.
https://www.savingforcollege.com/article/can-i-withdraw-contributions-from-a-529-plan-without-penalt....
It says, in part, "The basis portion and earnings portions are calculated using the following formula:
(Total Contributions / Value of 529 Plan Account) * Amount of Distribution = Basis Portion
Amount of Distribution – Basis Portion = Earnings Portion"
