Verify your build and software installation is OK by running a benchmark like Heaven and checking your scores against similar builds.
Your build should be more than strong enough to run TF2.

I will title this "When you do your AC task so well the cheaters still insult you after you're gone"

Good job Miggy

Thanks to the family for allowing us to participate

Thanks! I've banned those steam IDs and associated IP addresses.

They're just trolling idiots. Report and don't pay them any more attention.

Also, two of them played on serveme.tf at one point and both had static IPs. https://github.com/Arie/serveme/commit/febc44fb79e8e3abd18515380ac3baba46354378

I encourage all community projects and server owners to get rid of these idiots and find any possible alts.

If that's a graph of an idle connection, no, it will not help. If that's a graph of your connection while someone is downloading/uploading, yes, it might help.
If your connection always looks like this you might have a problem in your local setup or live in a very congested neighborhood with too many people on cable.
Don't upgrade to the superhub 3, it has the awful puma 6 chip inside

Had to take it offline because I could no longer update the outdated wordpress with security problems due to outdated wordpress plugins with security problems.

In-ear headphones should block out enough outside noise for your use case. Active noise cancelling at your budget doesn't seem feasible to me.

I'd try putting your TP-Link in the DMZ of your current modem first. You'll get double-NAT this way, but this might be acceptable.
0.6Mbit up between 5 people might still suck though.

A recent version of DD-WRT should work, configure the QoS according to this guide (HTB + fq_codel) https://www.dd-wrt.com/wiki/index.php/Quality_of_Service and make sure you set the down and up bandwidth of your connection correctly.

But if you can run DD-WRT, you can probably run LEDE as well and get cake QoS instead.

Correct. Or you could buy a wifi router that's flashable to LEDE like the TP-Link Archer C7. You'll still need a modem though.

Try IPFire. You'll need two networking interfaces.

So many cable modems with that piece of crap inside unfortunately, a list: http://badmodems.com/Forum/app.php/badmodems

And yeah, nothing can save you if you use one of those :(

Have you also added the following to the advanced egress/ingress options? (Might need a recent or custom build of LEDE).

diffserv4 nat triple-isolate

The first will deal with prioritization of the traffic, the second and third option will distribute bandwidth much more fairly between the different devices and destinations.

This accomplishes the same thing without all the extra configuration and without fixed limiting of the bandwidth for anyone.

I wrote a "little" post about that just now: http://www.teamfortress.tv/44322/shared-home-internet-without-lag-and-ping-spikes