I did lot of openwrt-projects, pls refere to my feedbacks.
1) Which hardware do you use for openwrt ? May be, I have it already available.
2) I usually provide custom image to be flashed to device, but I can also provide script to adapt standard image. _OR_ I provide simple GUI for user to set up vpn-functionality.
3) I can provide setup of open-vpn client on openwrt, _OR_ wireguard. wireguard should be better choice for your voip. I can also do server setup, of course.
Bid is proxy, until requirement specs settled.