Maybe I can help by narrowing the options down(Geist endorsements are hypothetical and not an in universe thing, neither he nor Harry controls Trismegistus). Not sure how much you've been keeping track of since I haven't seen you since chargen, my apologies if you know any of this already. Ancient Runes is a 3rd year elective in canon that actually does things in this setting, we may be able to get it online earlier through self-study or other methods such as asking Hufflepuffs and Geist for book recommendations. I have no idea how hard or otherwise it will be to get Astrology online, but it's described as Advanced Divination in the update titled Orthodoxy. It's classified as a subdomain of Astronomy in Summer's End though.
[] Child of the Stars(Astrology, Dark Arts, Ancient Runes) - A High Potential Build per word of Birdsie with considerable synergy(Exact words for the combination of Astrology+Ancient Runes: it has synergies that are just straight-up insane) and the current leader, it is described as a combination Geist would endorse(Ancient Runes+Astrology+Any Diverse) it focuses on the combination of Astrology and Ancient Runes, which will provide considerable esoteric ritual power once it is brought online, and with Mastery of Dark Arts and 25-30 Gnosis, can provide a 4th Trismegistus pick. However, it is completely possible this will require Human Sacrifice or Prayers to Dark Gods or some other morally dubious thing. Dark Arts in this setting... Sara put it like this, "It's kind of like how someone calling someone a criminal in the us can mean anything from their a murderer too they were caught by a cop with weed or going 55 in a 50 MPH zone too they published classified evidence that the government is illegally torturing people." Dark Arts also provides in conjunction with Geist's tutoring, a noteworthy source of immediate power(Such as enhanced physical stats and the Wound Transference Curse) without compromising standard human morality a sizable amount of the time, balanced by the fact that all witnesses to any usage of Dark Arts must be Trustworthy(Which probably means Hufflepuff) or dead.
[] The Lights in the Sky Are Stars(Astrology, Ancient Runes, Charms) - The Closest competitor to Child of the Stars, this substitutes Dark Arts for Charms, giving us what's basically a free class in our core group of classes. However, Charms is significantly more general and broad compared to something like Dark Arts or Ancient Runes. It provides a harder path to a 4th Trismegistus pick that is likely to be a bragging rights reward if we reach it, via Archmastery of Charms and Astrology combined with effort and 25-30 gnosis. There's less immediate power/charms grows slower than Dark Arts, but there are academic advantages and the immediate power we do get isn't illegal/can be used in front of witnesses.
[] Mini Dumbledore Pisses off Snape(Alchemy, Potions, Transfiguration)- 3rd place, this build goes for production of a Philosopher's Stone, has Geist's endorsement, basically gives us two free core classes, potions for subordinates/ourselves, excellent transfiguration skills, and the Physical Enhancements Alchemy provides once we get them online. It's named like this because Dumbledore is Archmaster(6 on a 7 point scale) in nearly everything. He's Grandmaster level in Alchemy(7 on a 7 point scale) and a few spells.
The Others below this point have 3 or less supporters
[] "No Need to Call Me Sir, Professor"(Care of Magical Creatures, Potions, Herbology) - This one is about potions and bioscience. Useful for buffing ourselves and subordinates. It's synergistic with Harry's ability to speak to mammalian animals and Geist would endorse this one. The inventor of the Basilisk was an example of a Care of Magical Creatures Grandmaster(Skill rating 7/7). It makes 2 of our core classes into freebies. Currently has 3 supporters.
[] Rune Knight(Ancient Runes, Diffindo, Battle Magic) - A build focusing on immediate combat power with whatever fraction of the esoteric potential ancient runes possesses without Astrology, that hasn't attained much traction and I'm literally only putting it here because it's my bugbear. It seeks to develop exotic and powerful applications of the cutting charm via the immense growth from a single spell as a pick(Predicted to be able to sever individual veins in a target if you spec for precision in year 5 by the GM or destroy walls otherwise which is impressive because that's the Gouging charms job). Once the build attains Ancient Runes mastery, it seeks to leverage that via Wordless Still Diffindos to inscribe functional runes in fractions of a second without touching them to surprise enemies in battle, a technique I refer to as Runes Anywhere or Runes Everywhere. The necessity of a Trismegistus pick in a spell to achieve this technique is contentious, however it provides considerable combat power via the mix of Ancient runes in combat time once that's online, battle magic's instincts/training, and transcendent diffindo skills. It banks on the principle that power begets power to do the rest.