Compared to CO2 and PM measurements, TVOCs are the most difficult to measure because they encompass a wide range of chemicals that trigger the sensor. Some are very harmful, e.g. aggressive paint and others (hopefully) harmless like sunscreen. Both trigger the sensor.

Additionally the sensors are only showing you correct absolute levels in lab conditions, e.g. using ethanol at a specific temperature and humidity. So in real environments, the absolute values are pretty useless.

We now use a newer generation of TVOC senor e.g. the SGP41 that is more index based and eliminates the issue of the absolute values.

We also observe that temperature and humidity can influence the TVOC measurements.

All in all, TVOC is good to detect spikes and check what is causing these, e.g. use of aggressive cleaning chemicals. However the absolute levels in e.g. ppb should better be ignored.

Yes, it is normal that fresh air flushes out chemicals and you can see this improvement. We do like the SGP4x series from Sensirion for TVOC sensors.

Are they accurate enough for me to ensure I’m keeping VOC levels down while 3D printing different materials?

Yes I think they should be suitable for that.

That could be an excellent use for sensors that only give reasonable relative measurements.

FWIW, TVOCs can collect together and move around in "waves", your indoor air is not just one big evenly distributed "soup". The sensors also have lots of factors that affect them, such as humidity - you can usually find more info in the docs for the sensor part.

A better approach would be to sample consistently and use that to build a graph showing TVOC ppm over time.