Shadowsocks and other bridges are used to circumvent censorship (by obfuscation your connection to the VPN), which you mention isn't the case, meaning you cannot compare the two in the same sense. One if for protection against decrypting traffic with quantum computers, and the other is protection against blocking the connection to the VPN.
Personally, I would choose wireguard w/ the quantum tunnels since I like saturating my 2gbps line, but again, this depends whether or not you want to "obfuscate" your connection to the VPN from your ISP - something which, from the options you said, only OpenVPN w/ Shadowsocks would do.
tl;dr unless you want to hide from your ISP that you're connecting to Mullvad, just use wireguard. Otherwise use shadowsocks and OpenVPN