Yes and no.
You don’t *need* to use Asuswrt_merlin firmware but you
need to «trick» your Asus router to install Entware and using partitions on an external drive by
booting with an especially prepared USB drive in it. You have to be familiar enough with ssh and the commandline to then install the correct package from here (probably “nextdns_nohf”):
https://repology.org/project/nextdns/versions
It’s more complicated and «advanced» than flashing it with Asuswrt_merlin, and it wouldn’t be “stock firmware” anymore, so that’s why I answered both yes and no.
If you hold out to for the next firmware update of asuswrt we should get DNS Filter back in stock and you can do much of the same as the nextdns cli there.