If you're using pre-compiled images, OpenWRT has a package manager and repo with OpenSSH in it. I used to do something like this[1] to install and update OpenSSH outside of internal flash, before deciding that exposing SSH on my router was a bad idea in general.

If you're compiling OpenWRT yourself, you can easily just switch them out in your base image, too.

[1] https://openwrt.org/docs/guide-user/additional-software/extr...