>>583767171996: RBY/FRLG + RS/Emerald + Colosseum
1999: GSC/HGSS + DP/Platinum + Ranger
2001: Gale of Darkness + Shadows of Almia
2003: Guardian Signs
Around 2006-2008: Omega Ruby & Alpha Sapphire (in an alternate world)
From here, the exact years get a little more complicated. B1W1 are supposedly about 10 years after Diamond & Pearl according to the Teraleak, but Sun & Moon are about 20 years after Red & Green, and math doesn't quite add up unless you seriously stretch your definition of "about".
2009-2012: B1W1
2011-2014: B2W2 + XY
2013-2016: SuMo + USUM
Sword & Shield were the first games in the series to have ongoing events that could be accessed in the overworld, with these events being announced with real-world dates. This could imply that these games and their events occur around when they were released in real life, just like RBY and GSC originally did.
Probably 2019: Sword & Shield
Probably 2022: Scarlet & Violet
If this claim about Z-A taking place 5 years after XY is actually true (I have yet to see an actual source), that would put it either right around Sword & Shield, or slightly before.