FWIW I think you did the right thing. He probably would have dragged on for a little longer suffering even more pain until he curled up somewhere and died. You cared for him in the best way possible knowing his understandable feral trust issues and when things looked bad you sought a professional opinion.

I know a few vets socially and they all DREAD the "please keep moggy/Rex alive for as looooong as possible" brigade and are reluctant to suggest putting an animal down unless they ABSOLUTELY have to because while they love animals they also feel that it is unfair for us to apply human values to animals and that when it is time, it is time to ease their suffering. It is people being inappropriately sentimental and impractical which means they are wary of doing best for animals.

You put Blackie first and your own very difficult, complex and sad feelings second. Huge respect to you from this tiny corner of the interwebs.