I had an Asus RT-AC68U for years and the process for updating firmware was easy. Check on update tab in admin panel and it just did it. I just replaced it with a RT-AC86U because its bettter/faster ...
I actually used to buy Asus because their router hardware was pretty much Tomato with some extra shit on it and I could forward ports to my Snort VM. But I switched to Synology which allows the same ...