How exciting! Robert Brand (1725-1799) was my GGGG grandfather! According to my info he married Elizabeth Gilespie (1781-1806) (sister to Mary who married Thomas Carlyle.) I think he may have married her on his death-bed!
Jean/Jane (b1797-1876), their second child, married Thomas Paterson (b1801). This is where it gets complicated: my parents were second cousins before they married:

Their (Jean/Jane and Thomas Paterson's) eldest daughter, Mary (1820 -1893), married John Tennant (b1816) - they are my maternal GG grandparents.

Their (Jean/Jane and Thomas's) third daughter Ann married James Emerson. They are my paternal GG grandparents.

I have more details if they are of interest, linking with the families of Carlyle, Murray, Johnson, Emerson, Johnston, Nicholson on my father's side and Hunter and Davidson on my mother's.

I'd love to hear from you and compare notes! But, like you, I have no info prior to 1725. Jose