You are right that the
post-exilic nation of Israel in Ezra, Nehemiah, Zerubbabel, and Joshua's time in general was
not practicing idolatry. God had revived the people in Jerusalem in those days with a true fear of the Lord, as Haggai the prophet wrote. They had learned a hard, 70-year-long lesson against practicing idolatry, or there would be consequences coming from God.
But by the time the year 19 BC rolled around, with the "iron" Roman empire in power, this lesson had been forgotten once again. (Boy, is humanity hard-headed!) Israel's religious leaders compromised their own strict Mosaic laws in Deuteronomy 8:25-26 by voluntarily making their own people produce and then use that abominable Tyrian shekel in their own temple's worship system; a practice which God so despised.
Every adult male who was paying their yearly Temple tax was required to pay it using that abominable Tyrian shekel or half shekel. And
no buying or selling anything in the Temple without that one particular coin. Plus, the married women of the nation wore a set of these coins
on the forehead of their headdresses, denoting their marital status.
No, Torah Keeper, I wrote that
the biography of the Sea Beast had begun back in 607 BC, and was on its 666th year of existence as John was writing Revelation somewhere between AD 59 and early AD 60. The "
mark" with the idolatrous image made by the Land Beast which gave homage to the Sea Beast
began back in 19 BC with the imitation Tyrian shekel production, minted at Jerusalem.
This "mark" lasted until AD 66, with no one buying or selling in the Temple without using the required Tyrian shekel. It was in AD 66 that the Zealots who hated their Roman governance launched the rebellion and began minting their own coinage as a sovereign, independent nation of Israel once more (which independent nation of Israel only lasted until AD 70).
The ancient biography of the Sea Beast did not always include that "mark" being used. The Sea Beast was meant to represent the conglomerate set of pagan empires (having those former "lion", "bear", and "leopard" features plus the final "dreadful and terrible" beast) that had subjugated Israel and Jerusalem ever since 607 BC when Nebuchadnezzar deported the first "good figs" back to Babylon for captivity (including Daniel). It was only toward the end of those 666 years of the Sea Beast's existence that the "mark" was imposed by the Judean Land Beast on those dwelling in the earth (
tes ges - those living in the land of Israel or worshipping at its temple).
The "mark" of the Sea Beast had a very short lifespan, comparatively speaking,
from 19 BC until AD 66. It was this period when the "clay" became mixed with the "iron" of the Roman empire by joining in idolatrous practices with the priesthood-required use of this Tyrian shekel.
Of course not. Quite the contrary; this was the period of "the beginning of sorrows" of wars, rumors of wars, plagues, famines, earthquakes in divers places, persecution of the saints by the Jews and then the emperor Nero in AD 64, etc., etc. All of this was even
before the "Great Tribulation" and its "Days of Vengeance" ramped up in Israel from AD 66-70.
LOL, no, I won't say it's Nero.

That is a rather common Preterist misunderstanding, unfortunately.
This is very simple. The number of 666 is called "the number of a
MAN" because it was
Daniel's statue of a man which represented all those 4 pagan idolatrous empires, plus the "clay" of Israel that finally became mixed with the last "iron" Roman empire. You remember, of course, that in Daniel 7:4, the first "lion" empire (also known as Nebuchadnezzar's first empire of gold) "was lifted up from the earth, and made stand upon the feet
as a MAN, and
a MAN'S heart was given to it." This first "lion" empire was the numbered beginning of the name of the Sea Beast. A name is usually given to something at the beginning of its existence. Same thing here with the number of the name of the Sea Beast which came into existence back in 607 BC, and had endured 666 years as of the time John was writing his Revelation.
So long as this interpretation consistently follows scripture right down the line, is this a problem?