I believe THH is very weak as a REMI. It has affinity for the serotonin transporter, which means it can technically compete with serotonin, but it's not otherwise clear what it does there or anywhere else for that matter. If it does inhibit serotonin transport appreciably, this could potentiate DMT to a degree by preventing the transporter from removing DMT from the synapse.
A more interesting effect of THH may be to induce long-term upregulation of the transporter, which may lead to a few days reduced serotonin transmission activity follows by an upregulation of other serotonin subtypes (as they compensate for the reduced serotonin in the synapse). Presumably it could be used periodically, like maybe once per week to maintain this effect. In the long-term, this may have effects which are in many ways opposite that of SSRI drugs. Hence, I'm inclined to call THH an "anti-SSRI", even if the acute effects may block some serotonin uptake.
A more interesting effect of THH may be to induce long-term upregulation of the transporter, which may lead to a few days reduced serotonin transmission activity follows by an upregulation of other serotonin subtypes (as they compensate for the reduced serotonin in the synapse). Presumably it could be used periodically, like maybe once per week to maintain this effect. In the long-term, this may have effects which are in many ways opposite that of SSRI drugs. Hence, I'm inclined to call THH an "anti-SSRI", even if the acute effects may block some serotonin uptake.