The new bundle preview interface is awful, seems like SBT would be a great fit for providing a better preview of the bundles as well, instead of a separate mod.

I agree, the bundle UI is really weird.
I planned to do something about it, but it won't make the cut for the initial 1.21.3-1.21.4 port.

Since I searched anyway, might as well post my results here:
Bundle mods available for Fabric 1.21.4 are…

• "Bundles beyond" shows all slots and gives a couple more controls options, like WASD/arrows.
• "Bundle inventory" basically turns bundles into virtual chests.