Pests
The amount of sciridae increased a lot (as expected, because the population should be about to collapse), but I also noticed that they were being accompanied by exactly 1 premature spider mite (drop in RH). Because my environment seemed to fall out of an equilibrium, I thought about using fine quartz sand (0.1 - 0.4 mm) and nematodes to hinder the adult Sciaridae from laying eggs and also actively hunt for their larvae, while being able to get back on watering regularly... In the end I actually ordered those - but decided not to use them because the process of drying out the substrate was already in its final stage [foreshadowing: bad idea].
Plants
Also, the variegated plant got cellular damage... most likely because my temp. and RH varied too much (VPD and temp. spikes were too extreme and rapid). [Variegated plants in general are less resilient and need a more stable environment].
Setup
Until now, I didn't use any fan so that I could keep the moisture and temperature up, but from now on, I will start to activate an exhaust fan at about 0,5 -1 air exchanges per minute. This should homogenize the environment but also decrease the temp. by 1-2 K, and the RH yet again by another 10%. It also allows the plants to transpirate more efficiently, and with that, normalize the VPD.
Living Soil
In general, when the pests are in control, I will yet again focus on the 'right' environment rather than battling those pests. With this approach, I can keep the moisture content in the tent the way I want to without worrying about anything else.