Richard GARLAND b. 1771 in Tiverton was still alive in 1851, living in Llandilofawr, Llandilo, Wales, a widower, described as a Gentleman and annuitant. He is shown in the same location in 1841 married to a Hannah, so I think this may eliminate the Tiverton Richard from your searches.
I note that he & Elizabeth named one of their children Francis Burrell GARLAND (bapt 27 Dec 1802, born 10 Oct 1802.) Perhaps BURRELL is a clue. I also note that a Bartholomew and a Humphrey GARLAND also married and were having children in Stoke Damerel at that time - brothers perhaps? Bartholomew was a shop keeper; Humphrey a labourer and Richard a cordwainer.
A Richard GARLAND was buried at Stoke Damerel 6 Nov 1816, aged 62 - making his birthdate ca 1754.
A Francis GARLAND was buried at Stoke Damerel 24 Sep 1836, aged 76, making his birthdate ca 1766.
A John GARLAND was buried at Stoke Damerel 10 Oct 1835, aged 85. He was Sexton of the parish of Stoke Damerel for 53 years.
The name was prevalent in the parish - it may be necessary to check other Plymouth area church baptisms. I have looked at Plymouth St Andrew for you - no luck there.