He's being sued by the other founders which means he won't get them to agree to let him sell shares before IPO plus six months.
Also the SEC will investigate (if they aren't already) as this particular financial instrument (blank cheque company / SPAC) expects that the target of the acquisition is kept anonymous to isolate from insider trading. These instruments often lead to extensive legal action after IPO and they have fallen out of favour at least in part because of this.
Also, if Trump attempts to sell a chunk of his holding, the price will fall sharply so he'd never realise near this value. See Tesla share price when Musk sells shares.