Sell at the valuation, e.g. 5% of $XX,000,000. Maybe offer them a 20% discount on the shares if you really want to sell. Otherwise don't sell. Not selling is your leverage, as they clearly want you to sell.
It doesn't sound to me like they clearly want the OP to sell. It actually sounds like the opposite. The OP wants to get out, and they responded with a lowball offer which, if anything, would motivate the OP not to sell.
His best bet is simply not to sell and to wait for them to make an offer.
OP is confused about the shareholder and employee roles, they are not linked other than through vesting and the shareholder agreement (which they have presumably signed).
He can easily quit as an employee while remaining a shareholder in the company. Those roles need not be connected forever and there usually are - vesting excepted - penalty free ways of stepping out of a company while you keep whatever shares you already have. Clawbacks in a situation like that are very rare.