Regarding the Banana Pi, I was looking at them for ages and someone said to go for the NanoPi over it as the support on the software NanoPi is better.

I guess it depends. The BananaPi guys work very closely with the OpenWRT people, if you notice they usually provide testing / dev boards to members of the community before releasing things, they commit code to the project and their routers are usually OpenWRT first. There’s also an upcoming OpenWRT router from them that has been designed in collaboration with OpenWRT developers.

NanoPi has Armbian which is fine and nice however if you want a router, great OpenWRT compatibility is certainly more important.

everyone says that eventually everyone ends up running OPNSense anyway

I have to disagree with this. That’s mostly hype and people who don’t know what they’re doing, there are good reasons do pick OPNsense at a medium size company with a large and complex deployment but certainly not at a smaller scale. Either way OpenWRT is highly modular and very well documented you can just install whatever you require.