Here is the funny thing. I didn't say that the ISP wasn't the bottleneck. I said a hardware solution to deal with that bottleneck would work and is what OP is really asking for. Now we know getting a faster speed is not an option right now. Which I kind of correctly assumed given OP's statement that his rural and that most people already know if their internet is slow buying faster internet would help without the need to ask in a tech forum. It's time to start actually answering the question OP asked instead of just suggesting 'buy more bandwidth, dere".
If OP wants an actual solution to his problem and not a lesson on parenting or "duh, just buy more bandwidth", he can reference my original post. And I also never suggested he buy any new hardware, it might be necessary or it might not. If OP has an old PC laying around, pfSense will run happily on that. Or heck, you can run it in a VM on your current computer if you really wanted to, but that get complicated.
Oh, and another benefit of pfSense is whole network ad-blocking. Loading ads is using bandwidth, get rid of it on every device on the network.
Consumer router QoS is absolute junk. In fact, QoS in general is pretty shitty at what op wants in his situation, but TP-Link's implementation(like most of what they do) is particularly shitty. You want something that can actually set and enforce bandwidth limits. Because even with QoS, it will still let one device take up all the bandwidth if it things that device needs it.
Now, you want to see something really cool, check out this:
View attachment 246520
Now, I know what you're thinking. You're thinking "is that the same router that OP has?!" Yep, it sure is. Now you next question is "what's with that layer of dust?" And that answer to that is this router just sits on my shelf unused. Why? You ask. Because it's a huge pile of ****. I know for a fact the QoS is absolute trash on that router because I actually used it.
So, the answer to OP's problem, now that we know what we know, is a better piece of hardware that can do proper per device bandwidth limiting. You know, like I originally suggested. Set the phones to have 512Kbps, they don't need more than that to make WiFi calls or stream music. They won't be able to stream video to their phones, but they shouldn't be doing that with other streaming devices in the house. Set each video streaming device to 2Mbps(which will force 480p video) or 3.5Mbps(which will limit it to 720p). Or another option is to limit bandwidth to a group of devices. Give each kid 2Mbps for all their devices, which is still enough to stream 480p video with a little to spare. Let them figure out how they want to use what they are given.