The children names in the will was, Richard, William, Thomas, Sarah, Ann and John, and wife Elizabeth was sole Executive.
Is this the order that their names appear in the will?
Usually children were mentioned from the eldest to the youngest, but it was not a hard and fast rule.
If this was in order of their ages, they would seem to have had 3 children before Sarah in 1779 - the earliest baptism found so far.
Have you found the other three baptisms?
Added:
I can't find any baptisms for the older children to Richard and Elizabeth - Richard,William and Thomas, at Aldridge, or indeed the whole of Staffordshire, (or Derbyshire which is next to it).
HOWEVER, there is a baptism of a son Richard in 1759, William in 1760 and Thomas in 1766.
These are all children of Richard and Ann, as mentioned before.
Ann 'the wife of Richard Wadson' was then buried in 1774 at Aldridge.
(This was Ann Heath as mentioned before - she married Richard in Jan 1758).
It looks like what has happened is that Richard has then married, (or not!), Elizabeth a few years later about 1776, and had another 3 children with her.
Do you know how old Richard was when he died in 1806. The transcript on familysearch doesn't give an age, and the registers can only be viewed on findmypast.
You say in your original post, 'born about 1750'.
Is that just an estimate considering the baptism of the children you had found?
Still not found a marriage for Richard and Elizabeth about 1776.