Nighthawk R7000p only giving ~half maximum download speeds on NBN Fiber-to-the-Premises

Hello, networking-noob here. I have spent the past 2 days trying to get to the bottom of this router issue and I am about at the end of my tether. Bolding for emphasis.

I am running my personal Nighthawk R7000p router plugged directly into the network-provided NBN (National Broadband Network, Australia) Arris CM8200 modem using functioning cables. Using this configuration the absolute best speed-test result I have been able to achieve is 29mbps down / 19mbps up using ethernet on my 50/20 FttP connection. In addition, tests show my upload speed dipping all the way down to near-zero and I get semi-frequent drop-outs on the 5ghz wireless network (possibly the 2.4ghz as well, but I have explicitly noticed it on 5ghz). 

While the download speeds have been just inside the realm of tolerable over the past year with the NBN (the Nighthawk worked perfectly on our previous cable internet connection), having recently taken to playing peer-to-peer networked games such as GTAonline my internet stability has become a massive source of frustration. 

Attached is a screenshot of my last several speed-tests. I have noticed no correlation between wired/unwired or the different wifi bands in relation to these massive drops in upload speed; the last 5ghz test I did for example (not included in screenshot) came back with 12(ping) / 30.86mbps / 0.14mbps

With my laptop plugged in directly to the Arris CM8200 modem I see an immediate improvement with speeds even slightly exceeding my connection’s rating, and using the ISP-provided (Optus) Sagemcom F@st 5366 TN modem/router instead of my Nighthawk router gives the same good result. This clearly indicates the issue lies with the Nighthawk, and while the Sagemcom technically functions properly I am loath to use it as my main gateway as it is locked down to my ISP’s very basic firmware (limited or no access to advanced features, horrible UI) in addition to its weaker wifi signal. It also lacks a bridge mode feature to allow me to even test whether my router will function properly using such a configuration.

Things I have tried:

Multiple factory resets throughout testing

Updating to latest firmware 1.3.2.134

Reverting to much older firmware 1.3.0.20 (pre Netgear Armor)

Attempting to “fake” bridge mode between the two routers using DMZ - inspired by https://yescrowd.optus.com.au/t5/Broadband-Telephony/How-to-effectively-quot-bridge-quot-the-Sagemcom-3864v3-CABLE/td-p/570602 , gave up upon encountering an error at step 3 “On your MAIN router” and not really understanding what I was doing

Turning on and off QoS

Turning off Traffic Metering

Checking Access Control is not turned on (it wasn’t)

Holding my head in my hands because I don’t know what else to try

It’s taken the better part of 48 hours to educate myself enough on my network to identify the source of the problem clearly enough to make a (hopefully) informed enough post about it.  <--- The legibility of that sentence sums up where my brain is at right now.

Any further help from here would be very much appreciated !