Byron seems to comment directly on his own work in the lines — a. “from these our interviews, in which i steal / from all i may be, or have been before . .” b. “ten thousand fleets sweep over thee in vain . .” c. “without a grave, unknelled, uncoffined, and unknown.” d. “ —and what is writ, is writ; / would it were worthier! ”

Byron seems to comment directly on his own work in the lines D. "-and what is writ, is writ; / Would it were worthier!"
What he is trying to say here is that what's done is done - he wrote his piece and now it is published and done, so he cannot change anything about it not, because it is over. He only wishes it was better, but now it's too late to change it.