https://www.sec.gov/data-research/sec-markets-data/financial-statement-data-sets

fred data
+ bonds

request for form 5500
https://www.dol.gov/agencies/ebsa/employers-and-advisers/plan-administration-and-compliance/reporting-and-filing/form-5500

https://www.reddit.com/r/quant/comments/1g0gawn/comment/lsc8nuz/?context=3