Install SmartTubeNext on your client. As others have warned, many have tried using DNS - and many have failed.
You had asked about addon's to run. IMHO here are some ideas:
First, Asuswrt-merlin on it's own adds some very helpful functions. Many more things to tune/tweak (in my case, the ability in the WAN tab to reduce WAN MTU to 1420 for T-Mobile gateways).
If you want to assign clients specific IP addresses, you can't beat YazDHCP. Doesn't even need a USB drive. Much more useful than the built in DHCP Server options. Plus you can export and import your lists.
The next step up - Entware. Great stuff. And needed by various addons. There are lots of apps, libs, etc. available (opkg list > opkglist; more opkglist). Careful, easy to get carried away!!!
For keeping an eye on your network and router - here is a few: (and use AMTM to install them)
scribe/UiScribe - nice way to keep the log clutter under control
scMerlin - light weight, nice Web tab that shows memory/disk usage, process running) and bonus - restart many of the services running.
LED control (to turn off and on those flashing router LED's at sunset/sunrise)
RTRMON - nice script that gives you lots of detailed information about your network and router
If you want even more monitoring of your network (level 2)
spdMerlin - runs a speedtest at set times and lets you know how "fast" your wan is. YMMV
connmon - a nice occasional ping thing that lets you see latencys on your WAN
vnStat - lots of link and network level statistics and information on your WAN
(these do use wan bandwidth - your decision)
For the next level up - really want to tune things (warning - getting to some "expert level" stuff)
unbound Manager - very nice addon that lets your router be a real DNS recursive server. Believe me, trying to install unbound (the actual application) and ANYTHING can be maddening. This app just - makes it work. Ahh, if only
@Martineau would port this to RaspberryPi OS, he would be legend.
Note that I am not including some of the other addons like Diversion, FlexQos, VPNMON, Wireguard Session Manager, Skynet, AddGuard Manager etc. These are best left to specific issues and needs. Do you want to try to tune Qos? Need a VPN and ways to monitor and manager? If so, read many of the forum threads and decide if you want to jump in.
Last, for some fun with your new AX88U Pro - check out AMTM's Router Games ;-)
ps: If your installing an drive (any kind, SSD, USB stick) be sure to enable "dc" in AMTM. It will run a filesystem check of the drive at boot/reboot time. Be sure to enable it. You have been warned...