Internet speed is fine but when streaming bitrate keeps spiking for no reason

Pretty standard techniques for troubleshooting networks. You use ping an tracert.

Now with as bad as you are abusing your machines I would test from a different machine but you can do it from any device.

Your first step it to run a continuous ping to your router ip. This is to be sure there is no problem inside your house. Next run a continuous ping to some common ip that is very close. 8.8.8.8 tend to be a server in every major city. This is google DNS and they use duplicate ip addresses and you machines will use the closest one.

What you are looking for is very consistent ping times. You will always see some variation but it should be small. Spikes of 100ms or more indicate a issue....although 100ms spikes cause no problems for you average application it is just bad for video and games. Your goal is to see if there are ping spikes that correspond to the issues you are seeing in the bitrates.

If you see no issues in the ping you have to suspect it is related to twitch or your software. There could be problems in the network between your ISP and twitch uses for their ingress servers or there could be a issue in the path. None of this you can really do much about.

So lets assume you see problems with the ping that correspond. You next run tracert to 8.8.8.8.....you can actually use the twitch server if you really want to dig. What you do is start at the ip in hop 1 which is your router and run continuous ping commands. Slowly work your way up in the hops until the problem occurs.

The most common point is hop2 (unless you have multiple routers). This represents the connection between your house and the ISP. Problems here are generally related to cabling issue outside of your house. It could be the cable inside before the modem but that is not common. Problems here also unfortunately can be caused by over utilization by your neighbors on certain technologies. If the problem is time of day related this is even more likely.

You continue testing hop by hop until you get to the server. The farther away from your house you get the less likely you will be able to get it fixed. The lower number hops tend to be in your ISP control thing father are in different ISP.

Unfortunately some ISP have limited the respond they allow their routers so you may not get a response from ping or tracert from hops in the middle of the trace. Not much you can do but ignore those but it makes the testing much harder if there are a lot of them or if they are in the very early hops in the trace.