Prelude
When thinking about what a "seal" of a scroll (KJV book) might imply, the seven "seals" of Revelation 5-6 seem to read as a sort of prelude to the rest of the events portrayed in the following chapters. With that in mind, they arguably cover general events from John's day until the end of time and beyond. If that's so, the first four seals depict four horses that represents general events unfolding over the following centuries, events that increase in magnitude as the last "7 years" of Daniel 9:27 get closer (see Matthew 24:4-12) including the worldwide spread of Christianity in the white horse, as well as war, famine, plagues, etc. as represented in the remaining 3 horses.
The 5th seal spotlights Christians being martyred as something that significantly plays out as the future unfolds. The sixth seal, however, is somewhat more difficult to pin down. It could be describing the events at the end of the "7 years" surrounding what is commonly referred to as "The Rapture". (See Mt. 24:29) It could also be tied to events at the end of the Millennium, particularly the part about every mountain and island were moved out of their places. (See Rev. 20: 9,11; and 21:1) Or it could be both events with the "Rapture" displayed in the first part and Gog and Magog in the second part as depicted in the timeline above.
In particular, in verse 6:14, it seems to be an allusion to the end of the Millennium as is indicated in passages such as Rev. 20:9,11; 21:1; and 2 Peter 3:10 that talk about a time around the very end of the Millennium when the mountains and islands are removed and Earth's surface is recreated. That tends to tip the scale towards at least verse 6:14 being about something that happens at the end of the Millennium rather than at the end of the Great Tribulation during the rapture, like those who interpret the seals being a more specific timeline of the tribulation period argue for.
That alternate interpretation of the seals is something to consider, but I don't see that myself. It's not something explicitly stated in the text but rather has to be read into it just the same as any other interpretation. And when looking at the "Outline of Revelation" and "Patterns" sections on the Home Page there seems to be a strong case for the prelude interpretation put forth above. One last consideration regarding the sixth seal is there may be some interplay with that particular seal and the events at the end of the "Wrath" period in Rev. chapter 16:17-21. It all depends on how you decide to interpret things after careful consideration of the relevant passages. (See also Joel 2:31, Eze. 32:7, and Isa. 13:10)
A few more considerations. One has to do with the sealing of the Saints in chapter 7. Do the Saints get sealed for the GT or the wrath period? It seems to make more sense that this is talking about being sealed for the GT as there are passages that indicate that the Church will go through the 3.5 years of GT such as Revelation 12:6 and 14. (See also Mt. 24:15, 21, and 29) Again this would depend on if the Seals are interpreted as a general timeline of events or more specifically events of the GT.
Another is the phrase, "He that overcomes." There are several places where this phrase occurs especially in Rev. 2 and 3. It mainly has to do with rewards as the overcomer is the person who walks in fellowship with Jesus and does His bidding until he goes to be with Jesus in the eternal realm. It's separate from salvation as the scripture clearly spells out that salvation is by grace through faith and not of works. (Eph. 2:8-9) Rewards are given for our service and the more we serve the Lord in this life the greater our rewards will be in the next.