I very much doubt that all the records have been transcribed yet and just as many more may not have survived. His baptism may yet appear on records in the future.
Don't hold your breath, or imagine that there is a large quantity of such records waiting to be made available, because there isn't.
From the late 16th century IIRC the Church of Scotland was tasked by government with keeping records of all baptisms and marriages, irrespective of the denomination of the people involved. Naturally this did not happen, for many reasons including clerical negligence, difficulty of collecting information in remote areas or in very populous places, reluctance of other denominations to co-operate, and so on.
At the start of civil registration in 1855 all the Church of Scotland congregations were required to send their surviving registers of baptisms, proclamations and burials to the Registrar General, and these now form the bulk of the pre-1855 holdings of Scotland's People. Subsequently many registers - maybe even the majority - of other denominations' pre-1855 registers have found their way into the National Records of Scotland.
All but a tiny handful of the records held by the National Records of Scotland have been digitised and are available at
www.scotlandspeople.gov.uk. The few that have not are the registers of a few Free Church congregations where permission to include them has not been obtained, usually because the congregation no longer exists and it is not clear wh owns them. However the Free Church only came into existence in 1843 so its records will not contain the baptism of 'your' Andrew Miller/Millar.
Scotland's People also has all the records of the Roman Catholic Church in Scotland.
Apart from these, a very small number of registers from dissenting churches, and some records of the Episcopal Church, Congregational Church, Baptist Church etc still lurk in diocesan, local or university archives. But the emphasis is on 'very small'.
Before the start of civil registration in 1855, a significant proportion of baptisms is known to be missing from the surviving records. The further back you go, the more gaps there are. I have seen estimates that the missing proportion in the early 19th century is about a third.
The overwhelming probability is that Adam Mill*r and Sarah either didn't bother to have their children baptised, or had them baptised but for some reason or other no record has survived.
She may have been Sarah Murdoch; there is a record of the marriage of Adam Millar and Sarah Murdoch in Paisley in 1821, which would fit; and Sarah is not a very common name in Scotland.
It occurs to me to wonder if you have had a look at the Watt Library
https://www.inverclyde.gov.uk/community-life-and-leisure/heritage-services/collections/watt-library which holds the archives of Inverclyde Council.