The concern was that the body would be stolen (most likely at night after the Sabbath ended).
Again, Re: '~
The concern was that the body would be stolen (most likely at night after the Sabbath ended).~'
Perfect! Amen! So: "Have the tomb secured THIS THE THIRD AND LAST OF THE THREE DAYS he mentioned while he was still alive" so that it will be impossible for the disciples to steal the body '~at night[*] after the Sabbath~' and evidence that he did NOT rise on the third day would be lost.
[*as a variant in fact contains the words "at / by night".]
Therefore, the watch needed to be in place throughout Sunday. There is nothing here that says Yeshua would rise on the Sabbath.
Therefore, No! ~Therefore~, the watch needed to be in place "on the morning of the day after the Preparation" which was the Sabbath as you yourself say, already, and, '~throughout Sunday~' thereafter, until it could physically be proven that the body was still there and that "that deceiver who said he would rise the third day" the day before Sunday, never rose on it.
So there's nothing here that says Jesus would rise on Sunday and everything, to the very words of the text, prove that Jesus actually rose "on the Sabbath before the First Day" Sunday.