First off, silvertip is no more fragile than any other grade of badger hair. So choose the knot that sounds best for you based on how it shaves.
As for Finest being a step down from best, that is totally a YMMV thing. However, I personally love TGN Finest extra hair knots. They are a big step up in backbone from best, having the most backbone of any TGN knot. Plus if you were to choose the extra hair knots they should also be a step up in density, and both of these are good things for loading the soaps you were asking about. IMO Finest is also softer at the tips than best. Its not as soft as silvertip, but it is soft, and at worst has a just a hint of the skritch that your trying to avoid.
Silvertip Grade A is also a great knot, dense, soft, and with good backbone (though not as much as the finest). You may have to set the loft a touch shorter on the STA than you would with the finest for the same level of backbone, but it wont be uber short to get where you want to go with it. I dont think you can go wrong with a Finest or STA and I have both in my collection.
As for the loft, it will most likely be very high and floppy at 65mm. The trick is to have the knot bed on your brush drilled out deep enough that you can adjust the loft. As you lower the loft you will increase backbone and reduce bloom size. Think of it kind of like a triangle and shortening the loft (the longest side) with shorten up the other two shorter sides.
Whatever knot you get I would strongly suggest testing out several different lofts before you choose one to finalize. You can do this by making shims for the knot to set on (I use cork but anything that wont rot will work), and applying just a dab of silicone adhesive to temporarily set it. The advantage of this is that you can use test shave the brush and then adjust again with a different shim. Just use gentle pressure to pull the knot out, and repeat the process until you find the loft and face size (factoring in the different blooms) that works best for you. When you have the loft tuned in, just seal with more silicone adhesive or the more popular and more permanent epoxy.
"Loric=Ernest Shackleton of SBAD" - Krona Kruiser