He IS sacrificed "once for all". The offering made to God of the Body and Blood of His Son, Jesus, from the altar, is happening in Communion with the actual Crucifixion in the eyes of God. Eternity is timeless. What will come has already taken place in the eyes of God. What has gone is taking place this very second. In short, our sins today are contributing to the crucifixion that took place 2000 years ago in linear time. In God's eyes, it's still happening...
I'm not sure I understand your concept of time in perspective to God. Time is a created dimension - I think that much can be agreed upon. Also, God exists outside of what we know as the created dimension of time. As a result, God can view our time dimension at any point along its linear path. Also as a result, God is omniscient, and He alone knows the end from the beginning (Isaiah 46:9-10).
From OUR perspective, because we exist within it, there is time past, present, and future unknown. From GOD'S perspective, there is a plane of events that have a predetermined beginning and end, but which do not limit His actions the way time does ours.
With that in mind, yes, eternity is timeless. However, the crucifixion is not, because it was a specific event that took place in our linear time. Hence the Hebrews "once for all" - Jesus' sacrifice took care of all past, present and future sin. The RESULT of the crucifixion is of course timeless, because it is of eternal consequence.
In God's eyes it is NOT still happening, as you put it. In God's eyes, it is finished, in fact it has been since the creation of all things (Rev 13:8), because if it were not, it would be of no eternal significance whatsoever. That RESULT is what is timeless, not the actual event.
Hebrews 10:12-13, "But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God; From henceforth expecting till his enemies be made his footstool."