Hello Michael,
I'm sorry to be the bearer of bad news in your searches again, I don't think your line is connected to Hopkin Sims 1777
(unless there is a connection even further back but the important thing is that we can try and decipher what the correct lineage is and help each other as much as possible.
I think this is the lineage you proposed in your updated message:
-Hopkin Sims 1777
--Hopkin 1814 (by Margaret David) (+4 siblings)
---David 1841 (by Gwenllian Davies)
----John Cavassa 1858 (by Mary A. Winters)
-----Rosanna (by Alice Walters)
------ (Your Mother)
------- Michael Sims Myers (Yourself)
Unfortunately John Cavassa Sims I strongly believe through documented records is not related to the lineage proposed above.
This is the lineage I have evidence for if it helps. (sorry, but I hope it helps you with your searches)
-Hopkin Sims 1777-1868 (parents unknown)
--Hopkin Sims 1814-1890 (by Margaret David) +4 siblings
---David Sims 1842-1917 (by Gwenllian Davies)
David Sims 1842-1917 married Prudence John (as hanes teulu said) and had 8 children all in the Llansamlet area. (Prudence, David, Elizabeth, George, Winifred E, Thomas, Hopkin and Benjamin John.
Both David and Prudence are buried in Llansamlet.
So i'm again sorry to say I can 100% rule out Hopkin 1777, Hopkin 1814 and David Sims 1842 mentioned above as the correct lineage of your ancestor John Cavassa Sims.
On a positive note - there are lots of David and Hopkin Sims in the Llansamlet, Cwm and other surrounding churches. Sims is quite a popular name for the area. But as of yet I have been unable to connect John Cavassa Sims to any David or other Sims in the area, but if you do by any chance come across some leads that you think could be strong ones for his father/relative in the area - I would be happy to help you as much as possible and share any information I may possibly have on them from the church records.
Apologies again, as this may be not the news you wanted but I hope it helps in your searches and you never know, we could possibly be rellies connected further back than Hopkin 1777.
Kind regards and kind birthday wishes to your mother on her upcoming special day.