As i found out, there's one more thing to add, since i was experiencing dropouts some time after the OS booted and worked flawlessly.
I knew where to look and i found out the medtic (priority) of the default route(s) have been flipped again, so no backroute and no connection possible via home LAN IP.
Turned out that the USB-WiFi dongle which provides the stellarmate WiFi suffered from power issues on the onboard USB3 connector. So next time the power drops, the ifdown/ifup processes are run and remove/add the network's default route, which is added with a higher priority again.
After running the USB WiFi dongle off the powered USB 3 hub, the wlan0 and wlan1 interfaces became stable and the routing priority fluctuations were gone.

If i find a way to keep the system from adding a default route for a private network (stellarmate) on every ifup, i'll post it here, but this article should give you an idea about the core problem with dhcpd:

www.jeffgeerling.com/blog/2022/network-i...rity-on-raspberry-pi

A recommendable read if you use more than one dynamic network interface with Raspbian-based distributions.

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