If you visit the BTO website https://www.bto.org/volunteer-surveys/find-gap-european-breeding-bird-atlas-2 it will show you the coverage for your area.
As you delve deeper into your region you can produce a list of species and easily see what's been unrecorded. Take the Lleyn for example
A quick look at the list shows a couple of species that realistically must breed in the area and just need confirming.
So if you need any more background or help contact me at the BTO Cymru office.