Sometimes the native boys are just trouble, whatever you do, so buying new queens in is the only option.
From my queen-rearing something like 1 in 20 are 'flighty' and that queen doesn't survive long or if she isn't too bad, I stick her in a mini-nuc to give me brood for queen-rearing hives. Although there are beekeepers in the area which are close-by; several of them have had bees from me which helps with my genetics and as my colonies out-number the colonies I know of nearby, I have a little bit of a chance in supplying the drone gene-pool; there's some of my genetics in the hives near-by that come back.
I've never bought a Buckfast. I did buy a couple of Carniolans relatively recently and one was useless; the other did no better than my own girls for honey and behaviour was 'middleing' compared to mine.