The same conditions for mortal sin apply for the priest at Mass as for any other sin.
Was it gravely wrong? In virtually all cases, the answer would be yes since beyond the instructions for the Mass, the priest would be guilty of the sin of disobedience - grave matter for a priest.
Did he know it was wrong? I can’t imagine a scenario where the priest knows the Mass and reads from the Missal but has never read the GIRM.
Did he act with full consent? Here’s where we just don’t know. There are priests “of a certain age” that were poorly formed and who developed some very bad habits and may have a malformed conscience regarding Liturgy. Others just don’t think the rules apply to them.

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I generally agree with you you, expect for the way in which you apply the three criteria. For example, a priest might not know the particulars of a Mass (such as a new priest that has never celebrated with a bishop or that has never concelebrated). The matter isn’t just reading the GIRM, but memorizing it (just because a priest has read it doesn’t mean that the priest remembers its particulars).
Furthermore, there are some things that a priest may add (spontaneously, of course), such as a Hail Mary after the Prayers of the Faithful (as long as this doesn’t become a habit, and is honestly spontaneous).
Furthermore, there are some liturgical abuses that are worse than others. A priest adding a Hail Mary to the end of the Prayers of the Faithful for every Mass isn’t as bad as a priest that changes words in the Mass.
But, again, I think that you are right in regards to the three criteria, but not how you apply them to the Mass.