Unranked 6060Cold mod to increase Slash-bias.
Using a lower % Toxin bonus is not a good idea because the Kuva/Tenet bonus also increases "base damage", in turn increasing Slash proc damage.

While the Kuva/Tenet bonus opens up a mod slot vs normal weapons it also sadly reduces total Slash chance. A Kuva/Tenet melee with 100% base Slash will have a final Slash chance of: 1/(1.6×1.15) = 54% with 60%ToxBonus+R0 60/60Cold. That's about the same as a normal weapon with 70% base Slash: .7/1.3 = ~54% (R060/60 Viral)
If I were to make an educated guess about a threshold I'd say Kuva/Tenet weapons need more than 80% base Slash to be Slash-viable. Livia is the only Kuva/Tenet weapon with that much Slash, all the other Kuva/Tenet weapons have to go for Tox DOT or Burst DPS Viral builds.

Either pull out a shoot-y weapon to pause the combo timer or use an external source of Combo Duration like the Primary/Secondary Arcanes or Naramon Power Spike.