I think there are good reasons the California missions proceeded north, from San Diego being the first to the northernmost, Sonoma, being the last. Oh, they skipped ahead and backfilled, and while I'd have to look it up I believe they jumped far north to hold Monterey Bay (and belatedly, established Mission San Francisco and the military town of Yerba Buena to hold the great San Francisco Bay itself) early, but generally speaking they had to proceed gradually because one mission supported the next, and operating in California was quite expensive. They had to persuade not only a bunch of Franciscan monks but a bigger bunch of soldiers, recruited from the Mexican populace, to support them. I don't know if this has become such general knowledge that it goes without saying, but the mission system was basically a string of death camps--the hinterland was depopulated by the soldiers, sweeping the native peoples into custody at the missions, where crowded, worked, and exposed to Eurasian diseases they had no resistance to most of them died off very quickly. One reason for this was that the Russian fur traders who were the major threat to Spain's claim to own all of North America in the west (in a European context of course!) had the practice of winning over Native allies by enticement rather than attempting (generally) to overawe them with force--therefore loose California natives were so many security risks to the Spanish system. As long as the Russians and every other European power were all far far away, the Spanish simply left the whole coast beyond Baja California alone; they probably would have crept very slowly up the coast over decades and centuries if they had more of them--but in the couple they had since the early 16th century they'd gotten no farther than below San Diego; they never perceived opportunities for benefits worth the costs of proceeding to more actively incorporate the coast at any rapid pace. The missions of California of OTL were a response to a threat more than an investment and the rate at which they proceeded shows how much Spain, as an empire, judged it could and should allocate to cover their claims. In fact long before Spain finally lost her grip on continental North America completely (except for Florida, more an outpost of the Caribbean, and soon to be lost in its turn--having been taken from Spain outright by the British who later gave it back, only for the Yankees to extort it away again just a few decades later) they had already conceded that their claims did not extend beyond the latitude that is currently California and Nevada's northern border.
Yet the mission chain only got halfway to that latitude, and the last mission, Sonoma, was actually founded under Mexican rather than Spanish rule!
What all this says to me is that Spain either simply could not afford more of an effort or anyway did not think the cost of doing more would be worthwhile.
So first of all, your ATL Spain has to be stronger and richer in the eighteenth and nineteenth century to be able to readily afford an effort an order of magnitude greater than they made OTL on the northwest coasts, and to perceive securing her Tordesillas claim to the entire continent, at least on its western side, as worth that investment, and also to be able to hold on to at least some of her OTL continental possessions despite the revolutionary fervor that OTL kicked Spanish rule out of everywhere but the Caribbean (counting Florida as part of that sphere). Since the successor Latin American states proved notoriously weak and unstable, the fact that anyway all of them got rid of the Spanish overlords, none of them were reconquered by Spain, and there was no Spanish equivalent of "Canada;" no place where pro-Spanish regime loyalists fled to and made a stand on behalf of the Castilian monarchy--that is all very telling. All of that has to be reversed or at least some exceptions made for Spanish claims in the northwest to have any meaning left after the 1810s--somewhere in the Spanish Empire of the Indies, some region, specifically one with Pacific coast harbors, would have to be both rich enough to fund, staff, and otherwise support the California and northward forts and plantations, and attribute its prosperity to Spanish rule and thus support it.
So basically you'd need to revitalize the whole Empire of the Indies, or anyway strategic parts of it, which I guess takes us way back in the eighteenth century at the latest, show that one region that touches the Pacific anyway could have been richer using policies a royal administration could think up and be comfortable with, and then mobilize that wealth to support a whole string of coastal settlements.