The DL of St John fixes in form about 600 AD; the Gregorian does not fix in form until about 1200 AD.
We know this in part because the Dominican Liturgy (of the Order of Preachers) is fixed in form in 1200, based upon the Roman Missal in use at the priory, and because every archdiocese moderated its own liturgical missal, and the Carthusian and the Carmelite not long after; none matches the others. Plus the Sarum rite, the Celtic Rite, and the Gallican Rite are well documented; by comparing these, the West was positively brimming with local variations through the 1200’s and beyond. Trent abolishes anything under 300 years old, leaving the Roman, Dominican, Carthusian, Carmelite, Dalmatian, Bragan, Mozarabic, and Ambrosian liturgies permitted, and abolishing the Gallican and Celtic as having fallen from use, and the Sarum as too new (as well as a dozen gallo-roman and brittano-roman diocesan variant missals).
While there has been some development rubrically, the text of the DL of St. John is the text St John wrote. It’s further a redaction/reduction of St. Basil’s liturgy.
WHile the Roman Canon is essentially intact about the same time as St John’s DL, the Roman-Tridentine mass can not be said to be intact at that time, and the Gregorian mass evolves significantly. Popes kept changing stuff… and not every diocese kept it the same.
As for the focus: The Byzantine liturgy is dual focused…
- The representation of the Sacrifice of Christ
- The education of the faithful.
This latter element is generally downplayed in the gregorian mass, especially during the dark ages.