So how do you get a third party presidential candidate to win? By actually building a 3rd party first. Run candidates in local elections, get them on school boards and mayorships. Start locally, build a following and work up to state. Start winning an appreciable percent of state legislature seats and move on to congressional seats.

Exactly this. These cicada-like independents and 3rd parties that pop up every four years to run for POTUS accomplish absolutely nothing, other than acting as a spoiler to one or both the two main parties. Some, like JFK, are acting in an ego-driven fantasy world with their best possible outcome being bribed with a job or money to drop out of the race, while others such as Stein, are intended specifically to achieve a spoiler effect targeted to pull votes away from one of the two dominant parties by playing on wish-fulfillment fantasies of susceptible people.

The only way to achieve real political power in our US system is to either (a) do as you suggest and put in the hard work over years to build up a viable party from the ground up to get large and strong enough to replace one of the Big Two at the top, or (b) take over one of the Big Two from within, as MAGA has done with the Republican party, which is certainly faster. I’ve gone into how the left can do something similar with the Democratic party in other posts.