You're right that neither drug seems to affect serotonin to a large extend, but looking at interactions I did get Lithium + DXM (I'm aware that the MoA of DXM and ketamine is different and DXM affects serotonin much more than ketamine)
https://www.rxlist.com/drug-interac...al-and-lithium-carbonate-oral-interaction.htm
While the risk of serotonin toxicity seems to be extremely low and I agree with what you've said we can't rule it out completely (especially as Hodor said, because we don't really know a lot about the MoA of lithium) so imo better be safe than sorry
To be clear, I'm not aproving of someone with bipolar disorder taking K, just arguing regarding lithium.
Lithium is interesting when it comes to interactions. It doesn't increase any major neurotransmitter, doesn't interact with any enzyme responsable of xenobiotic metabolism, the only major way you can have an interaction is when certrain drugs (NSAIDs, ACE inhibitors and some diuretics) prevent the excretion, that can kill you or leave you brain damaged easily. The big reason I'm saying all these is I take 1200 mg of lithium, have been for years. I have taken sertraline, lisdexamphetamine, vortioxetine, and abused tramadol, tapentadol, morphine, codeine, methylphenidate, amphetamine, all the benzos, all in high dosages and while taking my normal meds. That led to fucking up my life for a year, but it wasn't an interaction, it's what happens when bipolar people abuse drugs. OP is asking if the interaction will harm him, and the answer is no, but the drug may, and because of personal experience it takes so much more to fix it than whatever fun you had. Ketamine increases dopamine levels by reuptake inhibition and indirectly as an NMDA antagonist. For a bipolar person, that can lead to hypomania or mania, which then makes you cycle. And once you're cycling (going up and down basically) it can take months to stabilize it.