I can see you play TruckersMP also - You may know me over from the ProMods side.
Anyway, your router along with ISP are weak choices. Also, many of the projects you use also use Cloudflare as their CDN you likely want to avoid Spark since they don't peer locally within NZ.
If you want the best performance to everything consider Voyager or 2degrees as they both peer in NZ to Cloudflare, and have good peering elsewhere. I can confirm that Twitch streaming is totally fine on both.
Now, your router. It is frankly terrible and will be limiting you. Gaming routers are really not good and this exact model is one which Asus actually offered me has terrible routing performance due to its software (I found in many cases I could not achieve Gigabit speeds). As you state you're a network engineer you will have no problems running better products. Consider buying a Ubiquiti UniFi Dream Machine and be done with it - not only does it have better routing performance (and can do Gigabit no problems) it has the advanced features you'd expect too and won't get in the way of your packets. Don't get sucked in to the marketing side of things like you have with My Republic's "Gaming" plan along with your router.
Voyager offer Gigabit for only $99 per month, one-off charge for a Static IP and IPv6 on request also. Twitch works great with them and they have great support also.