Random aside, if your issues are

I'm trying to learn networking...

 

In all of my testing, I've not seen speeds exceeding 100 Mbps (upload or download). My apartment has 5 flatmates, averages 7 - 15 devices during "peak" periods, and often has 3 people on video conferencing tools simultaneously. It's gotten to the point where one of the flatmates has had to make an exception and work in the office despite covid restrictions, and another is considering doing the same.  

 

Well that's a great environment to learn about "real world" office/home networking

Zoom tops out at 2Mbps up, 4Mbps down so 5 flatmates videoconferencing is 4*5=20Mbps down, 2*5=10Mbps up ...

Yet performance is so bad on your Gigabit that people have to work from the office...

I'd make a guess that your setup is an ONT plugged into a Wifi router and everyone connecting to the Wifi.

You say apartment, so probably in a building with other apartments, lots of wifi networks around, lots of radio interference.  Wifi will be the issue.

What you could focus your learning on is wifi - spectrum, modes (b/g/n/ac/ax/...), channels, scanning to see what channels are busy, ...

Note that the speeds written on the side of your Wifi router box (2600Mbps!  1300Gbps! etc) are lies.  You will never get that performance, even with a tailwind.  That's adding up the maximum symbol rate on each radio in the router and rounding up.  The old rule of thumb was your best case speed on quiet spectrum (no interference, only one device connecting, close to the Wifi AP) was half the number on the box.  So 150Mbps on "Wireless-N 300".

You'll be renting and there won't be ethernet ports in each room.  Old person story, when I was flatting in 2007 our landlord let us put ethernet ports in each room.  Depending on your apartment layout you may have the ability to run cables by the skirting boards to another part of the apartment to put an extra Wifi AP in, or just run straight into someone's PC.

Note that if you've got an HG659, its 5GHz 11ac wifi works but has a very weak signal compared with other 11ac routers.

I suggest find a friend (make a friend if necessary, I'm sure GZ is full of suitable people) in your area who knows this stuff, has some spare gear, and can help you out!  If you were in Wellington I have spare gear you could borrow/buy/have (don't ask how big the junk pile is).