AUSTIN, Tex. — Brian McMullen’s only plan on a Thursday afternoon in March was to watch as many college basketball games as possible. Parked inside his neighborhood bar and grill and eating brunch tacos, he followed one game on the restaurant’s TV screen while another streamed on his iPhone.

As the games played out, Mr. McMullen talked about his new life in Austin, Tex., where he had moved last October from San Francisco. Some of his biggest activities since then: reading the Harry Potter series for the first time and spending more than 100 hours completing “Dragon Quest,” a role-playing video game. He was also working out a lot, he said, and teaching himself a coding language to create his own games.

For his medium-term goals, Mr. McMullen said, he and his new wife had been planning to honeymoon in Japan for a month. But they decided to cut the trip short to fly to San Francisco to meet friends for the opening night of “Avengers: Endgame” in April, at one point discussing whether to rent out an entire showing. (They did not.)

“I’m currently taking time off for myself,” he said.

Mr. McMullen, 33, is part of an exclusive club: the semiretired tech millennial who left California after getting rich. Like many in this group, he is a newly minted multimillionaire who became wealthy by working for high-profile San Francisco start-ups like Uber and Lyft, which are now about to go or have just gone public. Once their wealth was assured, these tech workers quit the companies and fled California, which has the nation’s highest state income tax, at more than 13 percent, to reside in lower-tax states like Texas and Florida, where there is no personal state income tax.