If you only use the VPN for access to the camera, you can configure the firewall in such a way that even if someone deciphers the authentication and logs in using the credentials stolen from the remote Mikrotik, he won’t be able to establish any connection, i.e. the same way as if the VPN tunnel was yet another WAN interface.
So you can use bare L2TP without IPsec and even without the MPPE (use-encryption=no in the /ppp profile), or bare IPsec with enc-algorithms=null. Both can traverse NAT without problems, and it will be their only purpose.
But the attacker will be able to send you a picture from another camera if he takes the burden to log in instead of the remote Mikrotik.