Well, I can find six baptisms at St Anne Soho to Michael and Jane Cass (possibly matching the six children in the bio linked to)
Thomas and the two daughters already mentioned, plus
William, bap 12 Jan 1811, born 30 Nov 1810 (father erroneously transcribed as Richard on some sites)
John, bap 29 July 1813, born 13 June
Mary, bap 25 Sep 1822, born 29 August
Regarding those siblings, I'm still hopeful that Elizabeth who married Henry Townsend is going to be the right one. Father named as Michael Cass, Gentleman. Witnessed by William Cass.
There is also a marriage of a John Cass, Mariner, to Jane Frances Beard in Marylebone in 1845. Father Michael Cass, Deceased. Witnessed by Elizabeth Cass.
Is William the M.D. who is around later, married to Caroline from Cornwall?
On John's marriage in Marylebone, both parties have their residence in Portman Place, his is number 21.
The 1843 PO London directory has a Mrs. Cass in Portland
Terrace. But not found listed in the other available years.
Where were they in 1841? Are they missing from the census?
EDIT 1861 has Elizabeth and husband Henry Townsend in Kensington. Her brother William Cass, a GP, and his wife are visitors
https://www.familysearch.org/ark:/61903/3:1:3QHV-P3F7-B93S-8?i=631&cat=565670What are your thoughts? Do you think that Thomas Cass from Soho could be the surveyor?
John