There's no tether material nearly strong enough to withstand the forces from wind and the tether's own weight and drag at 30km altitude - plus 15km additional length for lateral drift.
The values for some materials are actually much higher than I remembered, with Kevlar reaching ~250 km. From a purely hold-its-own-weight perspective it seems like it might actually be possible!
Obviously there's a ton of other problems that wouldn't make it feasible, the first I can think of is the weight of a 50 km tether wouldn't allow the balloon to even take off. I'm sure you can think of another million reasons it wouldn't work.
The highest altitude recorded by a single kite is less than 5km. I understand the biggest constraint for higher flights is tether strength-to-weight ratio. Nylon is typically used - fishing line.
Tether diameter is another factor as the load from wind, on the tether alone, is considerable over such lengths.
Graphene does 50 GPa in bulk. A single atom sheet of it does 10 MPa on its own.
Take 75 metric tonnes of cable / space elevator ribbon.
That expressed as tensile pressure at an attachment point(s) is conservatively 7.5 GPa over an area about 10 cm^2. With sensible engineering, the entire craft (a very large one) would possess a web of attachment points across its perimeter to distribute forces more-or-less evenly.