I'm also 'stung' with the stale kernels even on the current beta firmware for my Slate AX. Even if proper btrfs-progs were in the GL.iNet repos BTRFS requires kernel 5.5+ for all checksum algo support. The sick irony is that my EOL'd Certa (GL-AR750) from 2017 has an approp. one but is built on OWRT 22:
root@certa:~# cat /etc/glversion && grep 'PRETTY_NAME' /etc/os-release && uname -a && nowutc
4.3.25
PRETTY_NAME="OpenWrt 22.03.4"
Linux certa 5.10.176 #0 Sun Apr 9 12:27:46 2023 mips GNU/Linux
20250902T090334UTC
root@slate-ax:~# cat /etc/glversion && grep 'PRETTY_NAME' /etc/os-release && uname -a && nowutc
4.8.0
PRETTY_NAME="OpenWrt 23.05-SNAPSHOT"
Linux slate-ax 5.4.164 #0 SMP PREEMPT Thu Jul 24 01:15:41 2025 aarch64 GNU/Linux
20250902T090410UTC
... & I'm still ignoring OWRT 23.05 is EOL as of last month.