Pests
The amount of sciridae increased a lot (as expected, because the population is 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.
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.