So obligatory, be aware of wireless issues in 21.02 and 21.02.1 given your using a WRT3200ACM.
Running 21.02-SNAPSHOT which has the fix currently, until 21.02.2 is officially release is possibly best for best wireless performance.
Adblock is an integrated solution with LuCI management and is compatible with dnsmasq or unbound.
AdGuard Home is an alternative option, it is however more heavy. The official package can be installed through opkg, management is outside of LuCi. It does some benefits of adblocking + DoT/DoH without additional packages. The recommended configuration would be to have AdGuard Home as your main DNS resolver and move unbound or dnsmasq to local/PTR only.
NextDNS can't say, never used it.
Most PiHole functionality can probably be achieved with the various options mentioned already, which are more native to OpenWrt with opkg and LuCi management. Not discrediting PiHole however, that is my personal view. I did used to use PiHole with multiple instances, but the lack of management for multiple PiHole instances to sync block lists etc (without running your own script/rsync) made it less robust which is why I moved away from it.