Borrow against the RSUs. If you have more than $500k, this should be trivial to privately arrange. (If less, idk. Also, not legal advice!)
How are the RSUs managed? Are they in a brokerage account? You may well be able to borrow from the broker.
You don’t own shares. You own a right to future shares. Borrow against that. (If you want to roll your own loan, sell a deliverable forward. Again, not legal advice!)