I'm currently working through 10 acres of buckthorn with varying success. I think I can help a bit.
raspberries and blueberries are not nitrogen fixers, raspberries may grow well enough to contend with the buckthorn, but blueberries likely won't.
If you killed the roots of the buckthorn or removed them you can pretty much plant anything, though if you simply cut it down you will have regrowth and a thick mess within a year or so.
Cover crop like clovers do seem to contend with the seedlings, I would recommend a mix of clover, some sort of annual grass(wheat, rye, etc), and something like radish or turnip. A deer food plot mix would work well so long as the grass included is annual.
The comfrey will help, but might not grow fast enough right away to suppress buckthorn seedlings and certainly won't compete with trees.
Fast growing trees like boxelder, elderberry, and black locust can help fill spacer temporarily and could be chopped and dropped later. Walnut seems to compete pretty well also. Weeds like Burdock have out competed buckthorn in places for me too.
Your second picture looks pretty clear, with very few seedlings or stumps around, you probably can plant whatever you want back there, I would guess you have other areas not as cleanly cleared and those are going to be tougher to reclaim.
Keep at it, we cleared most of our backyard and it has remained clear with many fun natives just showing up out of no where. After the buckthorn has been removed from the ecosystem it seems like alot of healing happens quickly. We've found hazelnut, highbush cranberry, dogwood, birch, maple, currant, raspberries, and even things like bloodroot, solomon's seal, and jack in the pulpit simply appear in recently cleared areas.
Here's a thread on some of my observations.