Settings at 720p stream?

I decided to stream on Twitch with 720p60 because I have 8 GB of RAM, ryzen 5 and gtx 1060 hardware.

My internet has a performance between 6-10mbits or so

I thought that I'm using a bit rate of 5000? Is that good or what would you say?

Sometimes it happens that the quality drops and then increases again what can you do that the quality always remains the same?

I would like to always stream at 720p60 and it shouldn't fluctuate with other qualities

Is it better to use a higher bit rate or a lower one?

What else do you have to watch out for?

My internet has a performance between 6-10mbits or so

What does your line only have between 6-10 Mbit?
Download? Upload?

Sometimes it happens that the quality drops and then increases again what can you do that the quality always remains the same?

Can depend on 2 factors. Your internet connection is too bad (ask above) or your CPU, if it is used for encoding, has too much load and therefore it can't encode properly. A Ryzen 7 and higher can help here.

Is it better to use a higher bit rate or a lower one?

You can also set a bitrate of 10,000 and it will not do you any good, regardless of the fact that 6000 bitrate is only possible for non-partners as long as your upload does not have the necessary capacity.

Many make the mistake of having a 6000 upload and setting a 6000 bit rate. That does not fit! Because the bit rate varies just like the upload.

For a 720p @ 60 a bit rate of 2500 is sufficient. I personally stream when on 720p, actually never, but also with 4500 or 6000 because my CPU handles it without any problems. For 720p @ 60, however, 3500 is usually sufficient.
I stream myself with 900p and normally have 2 birds with one stone at a bit rate of 4500.