If you feel like making yourself go literally insane and actively hostile to most of the people around you, you could go for a law track. Something relevant for undergrad (it doesn't matter that much AFAIK, history's a fun one that works.) and then on to law school. Good lawyers working for good causes can actually do a relatively high amount of good at the individual level. From union lawyer to tenants' rights, there's a lot of places that you can make a direct, visible difference in people's lives. That's a super hard road to walk down, though. It's a frustrating and monotonous profession composed mostly of reading the driest documents in human history, the pay is rarely good if it's something worth doing, a huge proportion of the people you'll be going to school with will give you a full-body feeling of visceral disgust, and it requires diligently working within the confines of a system that I'm going to assume you know is a wretched abomination.

Academics have like zero effect on the world lol

But like, fuck it, go to school for history. Whatever you want to study, just make sure there are faculty whose work you are interested in wherever you go.

Get a degree in Communism.

Make as much money as you can as far away from military contractors / the state department as you can and donate half to revolutionary orgs worldwide. Find a career that has enough downtime for you to organize during it or retire early to organize full time.

You can't really do an actual job that directly builds revolution under capitalism, at least not until there are parties paying professional revolutionaries. Not even working for a union fits the bill. You can do more for unionization by dropping $2k on retail workers trying to form their own union than siphoning from workers' paychecks to do "strategic" bureaucratic union work. You can make $100k more than a union organizer by going after a job in software or finance or some other pointless thing sapping industry. That's 50 unions you can jumpstart as just one nerd every year.

In terms of studying, this doesn't mean you can't minor in a subject you find valuable (like history) or even do a double major, taking an extra year to complete the higher level courses. But it's not really going to make you better poised, materially, to support revolution. In fact, there is a toxic poverty fetishization among the Western left that pushes them into an academic track where there are 10X as many grad students as professors (9/10 won't get what they want) or a nonprofit "mission-driven" track. We should resist that. It only makes revolutionaries more precarious. We need resources and organizing time to win this fight.

Honestly, its pretty hard for an individual to move the needle on systemic issues regardless of skillset. Conversely, almost any skillset can contribute to building alternate power and challenging those systems. Study what you have a passion for, then get involved in the work.

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I'm coming to something of a personal Renaissance of what "greatest positive impact" means. There are things like environmental sustainability which are so clearly very positive. There's also psychology that could help unwrench horrible that affect society. There's law to stand up to oil companies in court. There's medicine to help the sick. English to change the culture. Biochemical engineering to heal the lame. Neuroscience to learn about ourselves. Zoology to help animals. Plenty of majors get downplayed in importance. Plenty of the humanities are necessary when we're stuck in the ugliest skyscrapers and argue with thought terminating cliches.

Even if your goal is that you want to help, encouragingly, that doesn't make you unique. Your Marxist perspective might, but not the desire for a better tomorrow. This is to say that it still matters what you find interesting. Not all majors are created equal. It might be harder to use IT for good, but with contributions to FOSS not impossible. If you went HAM in making sure Unity felt pain because they ditched their place in the market and Gadot had a free Unity conversion tool, that would send a shiver down a capitalist's spine.

As much as I've seen in the adrenochrome factory, seeing my friends progress in their careers, and my work at the CIA, it matters less what you study and more how you make use of it. A lot of degrees can get you a 9 to 5 at an office, but it's possible™️ to think outside the box and use what you learned in school (and see it crumble and then learn how to repeatedly use bits and pieces of it).