At 01:20:00, Steve and Dennis are strolling through New Orleans' Garden District and decide to sit on a curb and talk. The house they sit in front of (the Brevard-Clapp House at 1239 First Street) was formerly owned by horror novelist Anne Rice.
In the scene where Steve travels back in time to encounter a group of slaves around a campfire, the head "shaman" is holding a wooden rod that closely resembles one of the time travel portals in the movie The Endless (2017).
Synchronic (2019) shares continuity with previous films by Justin Benson and Aaron Moorhead. It is mentioned that the titular drug was synthesized from a red flower grown in the California desert, referencing the plant smoked by members of a commune in the film The Endless (2017).
The DMT like drug used in the movie is a designer drug. In real world designer drugs do exist just by changing an existing drug molecular structure of an thus making it legal or making a new drug altogether. This has been an ongoing issue for DEA since it's a game of cat and mouse.