@GUIn00b Where to start? I setup and support this sort of thing professionally now. I also had this same headache at home until recently (fiber is so much better, so sorry it's so tantalizingly close.)
I'd highly recommend using a single router with dual WAN setup. I'm not sure about the particulars with that on OpenWRT, but there are some things to consider.
Do you know what the throughput rating is for your OpenWRT routers for QoS? Just about any router/computer will be able to forward packets at gigabit speed, but if they don't have an ASIC accelerator for QoS they can cause speed to tank when turning on QoS.
For example, my personal ER-POE will only forward ~140mbps with QoS turned on while the VMWare Edge or FortiGate devices we use for work start at ~300mbps, and our Juniper's will do full gigabit.
Setting up the firewall and/or NAT for your public IP addresses shouldn't change much if at all.
You can do true internet connection HA, but that tends to be so human-error prone that it ends up being less reliable than using a single router.