Hi Shamana
The age is out but I think this may be your Thomas in 1871 (same family as pinot found in 1851)
RG10/5733/12/16
Ellen SMITH, age 60, head, widow,born Llandygai
Thomas SMITH, age 32, son, widower,quarryman, born Llandeygai
John Smith, age 7, born Llanllechid
Catherine Smith, age 5 born Llanllechid
On Freebmd, there is a marriage for
Thomas SMITH and Catherine WILLIAMS Bangor, Mar Q 1863, vol 11b, page 719
and a death for
Catherine SMITH, age 36, Bangor, Jun Q 1869, vol 11b, page 407
also deaths for
Ellen SMITH, age 70, Bangor, Mar Q 1881, vol 11b, 370
John SMITH, age 54, Bangor, Jun Q 1869, vol 11b, page 405
Also found the marriage of John SMITH (B) and Ellin PIERCE (S) in Llandygai Dec 18, 1835 both otp
Smith was a very rare surname in mid 19th century Caernarfonshire

There is only one Smith family listed in 1841 Llan Degai
John SMITH, rounded down age 20
Ellinor SMITH, rounded down age 25
Thomas SMITH, age 3
John SMITH, age 1
Possibly Thomas was born in 1837 just before registration or he might have been registered late.
Also, I noticed a birth for a Robert Thomas SMITH at Bangor registered Jun Q 1841, vol 27, page 298. Have you considered/researched him at all? I can't see a death for him but nothing seems to show in 1851.
As a matter of interest do you have the date of Thomas Smith's death/burial?
Heather