Can I block clients connecting to the router after some failed attempts (password wrong)?
Yes, there's software and firewall rules that may accomplish what you desire. I'm not sure if any of the software is readily available on OpenWrt.
Are you asking about the HTTP GUI or SSH access (I ask because I have an SSH firewall rule you may be interested in)?
Is yours the one linked in the old forum? https://forum.archive.openwrt.org/viewtopic.php?id=62084
Both would be useful to have.
Finally is this enough to prevent any access from outside the network to ssh?
iptables -I INPUT -p tcp --dport 22 -s 192.168.8.0/24 -j ACCEPT iptables -I INPUT -p tcp --dport 22 ! -s 192.168.8.0/24 -j DROP