Regarding that time period after one receives communion - I received a response from my diocese regarding this and it was confirmed that while common posture is encouraged, each individual person does have the freedom to choose whether it is most appropriate for them to stand, kneel or sit at that time. For the reasons you state, I choose to receive early and kneel in prayer because this gives me more time to do so.
This has also been confirmed to Cardinal George in a query he wrote to Cardinal Arinze that this is appropriate
The new GIRM provides that the faithful should “sit while the readings before the Gospel and the responsorial psalm are proclaimed and for the homily and while the Preparation of the Gifts at the Offertory is taking place; and, as circumstances allow, they may sit or kneel while the period of sacred silence after Communion is observed” (43).
Regarding sitting or kneeling during the silence after Communion, some interpreted this as meaning that the faithful should stand until the last communicant had received Communion, which was contrary to the traditional practice of the faithful assuming their preferred postures immediately upon returning to the pews.
A controversy ensued, in which Francis Cardinal George, chairman of the Bishops’ Committee on Liturgy, queried the CDWDS: “In many places, the faithful are accustomed to kneeling or sitting in personal prayer upon returning to their places after having individually received Holy Communion during Mass. Is it the intention of the [new Roman Missal] to forbid this practice?”
The response was as follows: “
Negative, et ad mentem [No, and for this reason]. The
mens [reason] is that the prescription of [GIRM] no. 43 is intended, on the one hand, to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of Holy Mass,
and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free” (
responsum of Francis Cardinal Arinze, June 5, 2003 [Prot. 855/03/L], as published in the July 2003 BCL newsletter).