Wall of Draining (a tier-7 Wizard spell) works nearly the same in Deadfire.

But there is no weapon like Spelltongue.

I read somewhere that durganizing Spelltongue removes the action speed buff’s ability to work (and maybe the duration stealing, too); is this still the case? It was a bug as of 3.06.

Hm, I had no such problems - and I did durganize Spelltongue with that build. I did this with game version 2.0, so pre 3. And @Raven Darkholmeplayed it past 3.06 iirc and didn't report anything like that. I'm pretty sure he durganized the weapon, but it may be best to ask him directly (basically done with tagging ;)).