Dear folks,
on my two S1300 devices with 512MB of RAM and firmware 3.215, the free amount of RAM with ~500.000 activ Adguard filters listed in plug-ins menu is ~ 370MB.
The AX1800 Flint with firmware 3.214, 512MB of RAM and the same Adguard filter list has only 25MB of free RAM in plug-ins menu.
Firmware V4.x is making changes. Nginx memory leak was fixed in Beta V 4.1.0 Release 4 and release 5 is out now.
I do agree that Flint does like to use alot of memory, but it also does cache and reserve almost 20%
Running firmware V4.1.0 release 5
You might try using a different blocklist OISD Full list really all you need for the majority of situations. If you are running multiple list with redundant entries this can slow down you DNS