WASHINGTON—The U.S. Securities and Exchange Commission (SEC) on Wednesday proposed tightening a legal safe-harbor that allows corporate insiders to trade in a company’s shares, and other rules to improve the resilience of money market funds. The agency also unveiled measures to increase transparency around share buybacks and the complex derivatives at the center of New York-based Archegos Capital Management’s meltdown earlier this year. The slew of long-awaited changes mark a milestone for SEC Chair Gary Gensler who has outlined an ambitious agenda to crack down on corporate wrongdoing, improve corporate governance and address inequities in the markets. The changes, which are subject to public consultation, will affect a swathe of corporate America, from publicly traded companies and their top executives, to banking groups and asset managers including BlackRock, Vanguard, Fidelity and Goldman Sachs. The proposed tightening of “10b5-1” corporate trading plans in particular was pushed by progressives who have long …