As I use an OPNsense PC as my firewall/router and the RT-AX86U is now just an dual band AP connected using the 2.5G NIC, thought I'd ask for members thoughts and maybe why?
What is your use case for the RT-AX86U? If you don't need the extra features, options, and updates Asus-Merlin provides (or if they are not available in AP mode) then stick with stock firmware.
For some time now, I've been trying to "harden" my home network with fully isolated IOT and Guest networks using VLANs. I have two RT-AX86Us; one is the router and the other is either a mesh node or an AP. When the second RT-AX86U is setup as an AP, I use a script posted by Maxbraketorque...
I wouldn't worry about firmware update. Just set it once and forget about it. Custom firmware only because of Custom Scripts support. Everything else doesn't matter. If VLANs and GN are not important - stock Asuswrt.
IMHO, if your needs are basic such that plug-and-play solutions typically satisfy your requirements then staying with stock firmware would be adequate.
If you have networking requirements that are served best by Merlin firmware then that is the way to go.
I started using Merlin firmware in my previous router many years ago because I found ASUS stock firmware at the time as ‘lacking’, I guess that’s the best way to describe. Merlin firmware essentially took ASUS stock firmware and improved/fixed as best as it could but did it in a timely fashion. As a geek, that appealed to me.