Maths is a big subject too. And so is German Literature, or Engineering, or Roman History. Perhaps people aim to become competent rather than all-knowing.
I noticed this about the Gelug curriculum (a school of Tibetan Buddhism) -- Geshe:
The Gelug curriculum, which lasts between 12 and 40 years, centers around textual memorization and ritualized debate, and is invariably taught through the medium of the Tibetan language.
And 14th Dalai Lama:
In 1959, at the age of 23, he took his final examination at Lhasa's Jokhang Temple during the annual Monlam or Prayer Festival. He passed with honours and was awarded the Lharampa degree, the highest-level geshe degree, roughly equivalent to a doctorate in Buddhist philosophy.
The Dlaia Lama's training started when he was a young child:
According to the Dalai Lama, he had a succession of tutors in Tibet including Reting Rinpoche, Tathag Rinpoche, Ling Rinpoche and lastly Trijang Rinpoche, who became junior tutor when he was nineteen.
After you read an anthology like In the Buddha's Words, I recommend the answers to these questions, about resources for studying the Pali suttas online:
I think of this i.e. the Sutta Pitaka as "elementary Theravada doctrine, for lay people".
If you're interested in studying themes you might like Dhammafarer's Sutta Discovery series -- see SD 1 through SD 50 or so of the navigation bar on the right-hand side of this page. Not everyone agrees with everything he writes, but it is well formatted and annotated and cross-referenced.
I find that enabling the Pali translation/dictionary tools of Ven. Sujato's translations on SuttaCentral is helpful too (because part of my initial confusion is from using English words that have several meanings, and from different translators giving different English words, so being to understand what the Pali says is helpful; for that reason I like buddha-vacana.org too, e.g. here or here).
I wish I could tell you how to approach Mahayana. Some of the changes seem big (e.g. the Heart, Diamond, or Lotus Sutras seem to me very different from the Pali suttas), but some other differences seem to me so slight or incremental as to be hardly noticeable.
I think that the suttas (or the Chinese equivalent, i.e. the agamas) are seen as fundamental (or maybe elementary or introductory) by Mahayana schools.
Since finding the Pali suttas I've tried reading some Tibetan texts, and found that (with some knowledge or understanding of what Buddhism is, from the suttas) the Tibetan doctrine/text seems to be easier to make sense of than I think they would have without that.
Maybe you should beware of modern or semi-modern authors who explain things in their own words (e.g. the popular books which introduced "Zen" to American readers, 50 years ago and more) -- perhaps not as clear as studying earlier/original texts from each school.
Also you might be handicapping yourself by studying alone (without someone else), or by study alone (without 'practice'). The Buddha was explicit about depending on and learning from people you admire -- learning "morality" perhaps, in person -- and even if you only consider doctrine (as if it were a subject like Maths or Latin) people usually say it's beneficial to have a good teacher.
I did once read a (non-Buddhist) aphorism, "Don't just study the subject -- study the teacher."
Having warned against modern authors, the meditation instructions/descriptions I've found have tended to be modern authors (or e.g. the Visuddhimagga which itself must have been relatively modern when it was first authored) -- perhaps details or meditation practice were always meant to be taught person-to-person, or by example, or studied/discovered for yourself, anyway there are suttas which describe meditation, and maybe or maybe not that's enough -- anyway many forms (of meditation) are described in old texts, and there's more than one modern school as you know.
If you're trying to create a new habit (your having completed 10-day and month-long retreats, perhaps you're past that) maybe it's better to practice for 5 minutes every day than to practice for half an hour but only once a week.
Finally sila is important, fundamental. I've seen it described as a basis or root -- I wonder if it's a goal or fruit as well. Anyway not just book-learning, not just teachers, and not just meditation either, IMO.
Maybe not just negative too (e.g. "self-restraint" and "silence" and "harmlessness"), but also positive (e.g. "generosity" and "right speech" and the brahmaviharas).