What does 70 mean in Strong’s Concordance?
70 REPRESENTS COMPLETION, WHOLENESS, THE REAL THING.

There will be a transition period between the Second Coming and the Millennium; however, the time is not biblically given a specific number of days. Dealing with rebellious remnants, Satan is bound and imprisoned in the abyss — preventing him from deceiving the nations, resurrection of Tribulation saved, cleanup, organization, and the Judgment of Nations, etc., will take some time.

The actual Millennium is going to be peaceful under Jesus as King of Kings until the end of the Millennium, when there is a rebellion that Jesus terminates.  The Millennium is a peaceful rule with a rule of iron by Jesus.

The 75-day theory is not confirmed Biblically.

There is also a period of transition between the end of the Millennium and the Lake of Fire conclusion. Again, no specific number of days is Biblical.