I acknowledge that you have asked for us not to research Ann Phillips baptism etc, but if trying to find death/burial records for her, her age is quite important.
Apologies if I have missed something in the various threads on the same or similar family, but what is your evidence that Ann Phillips was born in 1784 in Lanivet? Or is it the best working hypothesis at the moment?
Personally, I would be unconvinced since she would only have been 13 when she married in 1797. Equally I would put Peter's birth date as late 1770s if not before. While it was legal for girls to marry at 12 and boys at 14, I have rarely found it to actually occur.
It may well be that both Peter and Ann died before the 1841 census, which does not make things easy.
The 1848 burial record of Ann Williams indicates that she lived at Herodsfoot when she died, not that she was born there, so maybe keep this one in mind.
Nell