Which MT router would be best for:

1Gbps/1Gbps ISP connection (FTTH) + secondary 300 Mbps/10 Mbps ISP connection (Cable provider) primarily for failover.

Device will function as the Firewall.

OpenVPN w/a maximum of 10 simultaneous users.

OpenVPN, is not a requirement its a disease.............

hapac2, hapac3, RB450Gx4, RB3011, RB4011, RB5009 will all handle 1gig connection.
Recommendation, RB5009 as best use of your $$.
If your budget constrained, hapac3 (but not for its wifi).

OpenVPN, is not a requirement its a disease.............

And why is that ?

Which MT router would be best for

You got many choices...
Take a look here https://mikrotik.com/products/group/ethernet-routers

It causes more issues here than its worth, just use WG.

WG is easier to setup but this does not make OVPN bad.
It all depends on the requirements.

I would agree if we were talking only for ROS v6 where OVPN could only run over TCP.
But in ROS v7 UDP is supported.

Yes but so now is quickset VPN

Yes but so now is quickset VPN

I don't use quickset.
So i don't know.