Check System Start Level (System-based Triggers) in Javascript Rules as part of "if-condition"

Thanks for the hint, this did the job (despite the problems of this approach reported here).

In fact I did have two rules for this in place already, so I just added a couple of more rules to keep the log clean: One rule at start level 70 (to deactivate a couple of rules during startup), and one at start level 100 (re-activating these rules).