I'll play devil's advocate for a moment regarding the timeline.
Most timeline information given in dialogue in the movies can be assumed to be rounded to the nearest fifth or tenth:
-In Iron Man 2, Tony says his parents have been dead for almost 20 years. This can be anywhere between 18 and 20 years (meaning the film takes place somewhere between 2009 and 2011).
-In The Winter Soldier, Cap says he's 95 years old. This can be anywhere between 93 and 97 years (meaning the film takes place somewhere between 2011 and 2015/2016).
-In Homecoming, Cap says he was frozen for 65 years. This can be anywhere between 63 and 67 years (meaning the first Captain America movie's present-day scenes take place somewhere between 2008 and 2012).
These lines aren't necessarily estimations of years in the context of the individual movies, but the people making the official timeline could potentially handwave errors by saying the characters rounded up the years. Cap being 95 may be more precise though, depending on whether he's been keeping track of his actual, chronological age.
The dates given in props may also be subject to being ignored or retconned for the official timeline, since unlike dialogue or onscreen timestamps saying what year it is, props are mostly just background details that are unlikely to be noticed by audiences, and can therefore be more easily ignored if the filmmakers are going with a sliding timescale.
The reason I'm saying this is because most of the movies don't actually have an explicit confirmation of what year they are set in.
Captain America: The First Avenger has timestamps showing it takes place in the 1940s, Iron Man 3 is 13 years after New Year's 1999/2000 and both Guardians movies have timestamps saying "26 years after 1988/34 years after 1980", giving them the precise setting of 2014.
But in the other movies?
-Ant-Man: The prologue is set in 1989. Then it cuts to "Present Day", but it doesn't say if it's 2015/26 years later.
-Civil War: The prologue is set in 1991. Then it cuts to "Present Day", but it doesn't say if it's 2016/25 years later.
All "Present Day" really means is that the movie doesn't take place too far in the past nor too far into the future.
Unless you're actively searching for background details/props that are only seen for a split second, most people aren't really going to notice if, say, The Avengers or Civil War are retconned to take place in any other year than 2012 or 2016, since there isn't anything in those movies, in big letters on the foreground, saying what year they are set in.
On the other hand, if Guardians 1 and 2 are retconned to take place in any other year than 2014, it's more likely to be noticed, since the movies explicitly show timestamps telling us it's 2014.
In conclusion, really, "8 years later" doesn't actually contradict much of anything in the movies, besides it being both 8 years after Iron Man and after The Avengers. Props showing the year are rarely shown onscreen in the movies either, or at least in a really noticeable way.
The only problem is Agents of SHIELD giving explicit years for whenever it (or The Avengers) is set, due to a lack of communication between the people working on the movies and the people working on the TV shows. If it wasn't for that, there would really be no problem with having a sliding timescale for the movies.