Your best option may be with other employers with more enlightened management and an appropriately large budget for retaining your valuable services.
Your current employers demonstrated that they are not willing to honour the spirit of the original contract they negotiated with you. Sure, they have some clause such that RSUs grants are awarded at the board's discretion, so perhaps they're off the hook legally. But similarly you can just terminate your employment and stop working for them if the deal is now not attractive.
That said, if you are still getting paid more working for this company than other employers, quitting for another job might not be your best move.
Can you compare what you are getting paid with other employees in similar roles who joined at different times? It is possible you may be getting under paid anyway.