The following is offered in the spirit of discussion and should not be taken as legal or tax advice.
As I understand tax exempt trusts, they have to have a "social purpose" outside of simple tax avoidance. And that usually means a whole set of governance overhead. As has been pointed out, in most pre-public startups, you are carrying enough carryforward liabilities that the "fair market value" calculation of the stock can be done in a way that you incur no tax liability
Reid Hoffman, Linkedin, https://www.saastr.com/the-pernicious-effect-of-dilution-in-saas-the-cold-hard-bloody-numbers/