There is no misconfiguration, it's just a timing issue. Most routers is not powerful enough to boot very fast enough to load all necessary service in time.
Your issue is coming from the fact, that upon boot routers allow users to start browsing as long as the internet connection is already up while it's booting other services. I've tested this on a VM with 2.4Ghz quad core power and still there is a minute or two before all services finishes its startup (like adblock and banip).
A probable workaround is moving the "DNS Directory" in the "Advanced DNS Settings" in AdBlock to a location that survives bootup. Unless the specified location is in a USB drive (which your device can't have), it will probably in a location that is in the flash portion of the router which in turn "not recommended" as there is a maximum a maximum read/write for any flash based memory. Remember flash mems of routers can't be easily replaced or can't be replaced at all. This limits the number of times you can save/delete/update a file.
In theory, by changing the said directory to somewhere that survives bootup, dnsmasq will load the list upon start the adb_list.overall which contains the processed domains for blocking by AdBlock. And this will make sure blocklist is loaded already even before AdBlock has finished it's startup process.
I've tested the above scenario and it works even before AdBlock had started. For the test i've even forced AdBlock to startup 60 seconds after the test router has booted.
The
/localstorage/adblock is a directory that is not part of the tempfs (/tmp folder) which survives a reboot (even a full shutdown/power off then startup/power on) of the test router.
I've opened one of the known porn sites (as porn is blocked network wide on both my actual and test routers) even before AdBlock was able to start.
With the above solution, you don't need to actually do a script to detect if adblock has finished running or not. Only caveat, you will need to store the "DNS Directory" in a location that will be probably part of the flash memory (like for example /root).