Set OPNSense default policy
As far as I remember, OPNSense has a default policy rule of "deny all incoming, allow all outgoing". If not, this should be one of the first steps to take.
Get your own VPN
If you can, you could use your own VPN service. I run a VPS for 6 € / month. If you can get your hands on something like this and install an openvpn server, you could always use that VPN for every connection.
So even if an attacker highjacks your connection somehow, he would only be able to see encrypted content and all content will be encrypted by a server you own and can verify / trust. You could also integrate this VPN into your OPNSense, so you'll be connected as soon as OPNSense starts up and has internet.
Regarding MITM attacks
Please someone correct me if I am wrong, but MITM attacks should generally be impossible when connecting to SSL backed connections, right?
These certificates (or rather the certificate authority the HTTPS certificates have been issued by) are generally trusted by your own operating system. Therefore, if someone wanted to highjack your connection without you getting some kind of certificate error, he would have needed to get his hands on a certificate issued by a worldwide trusted certificate authority and the address name matching the certificate.
I don't know what stingray is, but if it needs a connection to somewhere and the protocol to connect verifies os-trusted certificates, it should be safe.