This week especially, the pests got out of control. I noticed that the amount of scridae stayed the same, while being accompanied by exactly 1 premature spider mite. This was a signal to me that my environment - at this point - was not in an equilibrium and needed to change. So I decided to:
Order fine quartz sand (0.1 - 0.4 mm), which I will use as a top dressing (2 - 3 cm) so that the adult scridaes can't lay eggs and their larvae can't hatch,
as well as nematodes that will actively hunt for the larvae,
+ new fly traps.
Another aspect showing me that I needed to change something was that 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]). Until now, I didn't use the exhaust fan so that I could keep the moisture and temperature up, but from now on, I will start to activate the fan at 0.5 - 1 air exchanges per minute. This should homogenize, but also decrease the temp by 1 - 2 K, with the RH staying the same (I'll get back to giving water to the substrate in around week 4 because the sand should do what I tried to achieve with decreasing the substrate's moisture content).
In general, from now on, I will yet again focus on the 'right' environment rather than battling the pests. With this approach, I can keep the moisture content in the tent the way I want to without worrying about anything else - the nematodes actually appreciate a well-balanced climate that is also optimal for the plants themselves.