If you have the WireGuard config from Mullvad already, just edit your wg.conf files on client devices to route all traffic via the Mullvad servers. Basically replace all the values of the [Peer] block with Mullvad values.

If you can share your Mullvad wg config file and your wireguard-server config file here, we can sort this out together

Edit: actually since your only goal is to increase the Mullvad device limits, why not just use Mullvad-provided confs directly in your client WireGuard apps? Should be straightforward to do