Push it back farther - have an independent California republic
arise from a stronger Californio "liberal revolt" in 1836, which itself stemmed from California's status as a territory, rather than a department, in the Mexican Constitution of 1824, as well as secularization of the missions...
1836 (rather than 1846) allows for involvement and support by US citizens and interests, but at a time the "Californios" were still strong enough to (presumably) form a criollo elite, since it predates the Gold Rush emigration by a significant period; it also predates the conflicts between the US, Britain, and Russia over the Oregon Country and their various outposts; most importantly, the US border is far enough away in 1836 that a Republica de Alta California probably has a decade or more to gell as a homogenous and "free" society, as required under the Mexican Constution (which doesn't mean the survivors of the indigenous tribes still don't get indentured; this is not that a-historical).
Although having said that, if Alvarado et al are willing to come to terms with Estanislao et al in return for support against the central Mexican government and their loyalists (such as it, and they, were in 1836), the end result may be a slightly more tolerant society than historically, and a "California" that looks more like Peru than Chile, for example.
If the Californios are smart, and willing to accept both the "indios" initially, and are then open to emigrants from both (former) Spanish America, the U.S., and Europe, no matter their religion, in the long run, they can (possibly) both grow the population and keep it in balance between the (various) "sending" states, so they avoid a situation like Texas (or the Transvaal, later in the century).
When the Anglo-American conflict over the Oregon Country comes to a head, if the Californios side with the US, their independence is strengthened, and their boundaries may grow to the north; likewise, if a Mexican-US war breaks out over the Texas-Mexico border and the Southwest, if the Californios again side with the US, their independence is presumably assured, and the Republic's boundaries may grow to the east; which means that if the geopolitical situation is relatively stable in the mid-1840s, to the point that the Republic can possibly absorb the tremendous demographic changes the Gold Rush brings.
If the Republic sides with the US in the Civil War, sending troops (and specie) east, then the basic situation of large power and a smaller "allied" regional one (akin to Australia and New Zealand, presumably) is in place for the remainder of the Nineteenth Century, and presumably the Twentieth. Economic integration would follow (a transcontinental railroad being the obvious initial project), as would diplomatic (not political) integration/allignment, and I could see a mutual defense treaty being signed and honored very early (Nineteenth Century, even), but political integration would remain a concept of political scientists.
The end result will be that the 20th Century "Republic of California" is very much part of the US "sphere", but Spanish will remain an official language, and the "Rancho" mythos (Ramona, Zorro, etc.) will be as much a part of Californio cullture as cowboying is in Texas.
If the above series of events comes off, there are all sorts of potential butterflies, including where the Republic's boundaries are drawn; probably not the entire Pacific Coast, but something different than the historical US state boundaries. If (and it is a big if) the Californios reach into the Pacific, it is possible that Hawaii becomes part of the Republic's territory, for example.
There is the possibility of something closer to the elements of the Argentine-Chilean relationship here as well; it might not be as friendly as Australia-New Zealand, but still stop short of hostilities.
Interesting possibilities to consider.
Best,