You'd need to build OpenWRT from source with the mentioned patch for master or these two for the 19.07 branch.
For my ADSL2 use case(s), I just just set the following in /etc/rc.local to split the load assuming wireless usage will be either 2.4GHz ath9k or 5GHz ath10k at a time. If no DSL, split the wireless instead perhaps. YMMV.
# Manual IRQ balancing
# ath9k
echo 2 > /proc/irq/30/smp_affinity
# vrx200_rx
echo 2 > /proc/irq/72/smp_affinity
# ath10k
echo 2 > /proc/irq/144/smp_affinity