Last update for GT-AC5300 was: 2021/05/07 => 18.104.22.168.386.42643.
Is it really dead? ;(
A sad subject, you might say.
I also was and still am happy with the performance of the GT-AC5300.
It's a shame it was dropped, the lack of Asuswrt-Merlin support doesn't help either.
In theory there is talk of the beginnings of support in OpenWRT => https://openwrt.org/toh/start?dataflt%5BModel*%7E%5D=gt-ac5300 but I'm far from calling it "working".
I, too, plan to switch to the GT-AX11000 (still supported, last FW/BIOS from today! 22.214.171.124.386.45934 2021/11/10 , also with support in CFW - Asuswrt-Merlin).
Thank you for your feedback.