I think his defeat in 1970, and his appointment as Ambassador to the United Nations was better for his career being seen as having an experienced foreign policy. An alternative would be to have him as Gerald Ford's pick of Vice President in 74.
Bush would still become President in this scenario, the question then would be is Bush be able to fend off Reagan (or alternatively have an easier time doing so than Ford in OTL) and win the General in 1976?Oh alternatively, Nixon picks Bush as VP instead of Ford. He wanted Connaly OTL but Democratic leadership said it was Ford or bust. Nixon liked Bush a lot so maybe Bush might seen as moderate enough for Carl Albert?
This is possible to assuming he'd get re elected to the Senate in 1976.He could beat Reagan for the GOP nomination in 1980.