The Sunnah is that you write the phrase ‘sal Allaahu alaiyhi wa sallam’ in its entirety.
This is because it is a duaa (supplication), and the duaa is an act of worship, just as if it were spoken out aloud.
To represent it symbolically using the letter saad or saad –laam – ‘ayn – meem is not a supplication nor is it an act of worship, regardless of whether it be a statement or a something written.
For this reason, this symbol was not utilized by the first three generations for whom the Prophet sal Allaahu alaiyhi wa sallam bore witness with khayriyyah (goodness and excellence).
And with Allaah is the tawfeeq. And may peace and blessings be upon our Prophet Muhammad and his family and his Companions.