I think a fixed-time vesting over 4 years is pretty standard, starting from the moment of employment, with no additional terms. If they're not happy with your performance, they should fire you, rather than withholding shares. I think 0.6% of shares is quite low, and the tax reasons sound dubious.
> How do I make sure what I vest, I really do vest and they don't change the rules again?
The position of non-voting shares is quite precarious I think, because a voting majority in the company can simply dilute shares and render the STAK-owned shares less valuable. Besides that: just read the contracts, the STAK bylaws, and once you sign the certificate holder agreement it's relatively ironclad. I'd recommend getting some legal advice.