As to the "FIRST SEAL" / INITIAL "birth PANG [SINGULAR]" of "the beginning of birth PANGS [PLURAL]":
The "FIRST SEAL" is [parallel to] all of the following:
--the FIRST ONE of "the beginning of birth PANGS [plural]," the "G5100 - tis - 'A CERTAIN ONE'" Mt24:5/Mk13:5
--the "whose coming/advent/arrival/presence/parousia" 2Th2:8a of the man of sin (parallel to Dan9:27a[26]--"the prince THAT SHALL COME"... the "FOR ONE WEEK [7 yrs]"; this involves the description [of same] in Dan7:20-21 "whose look is more stout than his fellows")
--the rider of the white horse with a "bow" (often representing "DECEPTION"), Rev6:2
--Jesus will be opening the "FIRST SEAL" when He "STANDS to JUDGE" (Rev4-5, Isa3:13, Lam2:3-4 [the wording of which sounds rather similar to that of 2Th2:7b-8a]); the 24 elders are sitting on "thrones" and have "gold stephanos/crowns" (note Paul's wording of "IN THAT DAY" 2Tim4:8)
--the INITIAL "birth PANG [SINGULAR]," as noted above, involves both the ARRIVAL of the "time period" ['the DOTL' very long time period (earthly time period), on the whole] AND the ARRIVAL of the "whose coming" ['the man of sin be revealed' at the START of the 7-yrs, and involving ALL he will DO over the course of the "DARK"/"DARKNESS" portion of the DOTL time period (the 7 yrs; see also Dan7:7 and Gen46:2)--"the DOTL" is all earthly-located, whereas "the Day of our Lord Jesus Christ" involves when "the Church which is His body" is "caught up/away" into His Presence (to the meeting of the Lord "IN THE AIR" (i.e. NOT on the earth) and its "G4862 WITH - syn" word - denoting "'UNION' with, and/or 'IDENTIFICATION' with" Him)]
--2Th2 (chpts 1-2 actually) is basically Paul's corrective, informing them (who were being wrongly convinced that "the DOTL IS PRESENT," v.2) that "it will not be present if not shall have come THE DEPARTURE *FIRST* [the subject of v.1] AND the man of sin be revealed [the INITIAL "birth PANG [SINGULAR (see above)]" of many more "birth PANGS [plural]" that follow on from that FIRST one/FIRST SEAL]... [when the phrases "the Day of the Lord" and "IN THAT DAY" are found in the same context, it is referring to the SAME earthly-located time period, and chpts 1 & 2 have these phrases, which should be understood in conjunction with each other--as the SAME future "time period" of the 7-yrs, STARTING with the INITIAL "birth PANG [SINGULAR]"/opening of SEAL #1]
...consequently, I DO NOT believe "SEAL #1" has been opened yet (which is the INITIAL "birth PANG [SINGULAR; 1Th5:2-3 etc]" of "the beginning of birth PANGS [PLURAL]"). Not STARTED yet. *ONE THING* MUST take place *FIRST* before IT (that "time period) can be present to unfold upon the earth over the course of some time.