‘scholar’:
Recent published historical research on the origins of the Bible Student's movement later becoming known as Jehovah's Witnesses shows that the early roots were not of Adventism and mainstream Protestantism but the 'Millenarian (Age-to-Come, Literalist) movement with such prophetic study from the start of the 17th century.
No, you’ve made a category error. 🤦♂️ Millenarians (specifically the belief in Christ’s second advent bringing about the 1,000 year reign) are Adventists. You may be confusing the term Adventist with the more specific ‘7th-day Adventists’ or some other offshoot. And the connections between the Millerite and Bible Student movements are well established. And all of them are offshoots of Protestantism.
If you really want to get specific, I suppose you could say JWs developed via the Bible Student movement via Adventism via Millerism via millennialism via Protestantism, but it still doesn’t get around their Adventist origins via Protestantism 😂