If the fallen angels are locked up in Tartarus ready to be let out during the great tribulation, presumably when Satan is cast down to earth, who is tempting us now – apart from our own flesh and the world, which is bad enough!
Both 2 Peter 2:4 and Jude 1:6 say the fallen angels in Tartarus are being held for judgment, so they won’t be freed during the Great Tribulation. Satan has a demonic horde to assist him, but like you said our sin nature provides all the temptation we need. According to James 1:14 we’re tempted when, by our own evil desires we are dragged away and enticed. Then after desire has conceived, it gives birth to sin.
During the Millennium, Satan will be locked away. But those who dwell on Earth, still in their natural state, will require a daily offering for sin (Ezekiel 46:13-15).