I also recently purchased a C8F and wanted a spare battery. I went with the Samsung 50E. I was looking at post #36 in this thread:
All three of those batteries make good choices in my opinion but the Samsung 30T and Sofirn battery will give a little higher output current due to lower battery resistance. That’s really only a concern if wanting to use the max output for any length of time.
For me I wanted a little more capacity and will only use the max output on occasion so I don’t care if I have slightly diminished output. (I probably won’t notice the difference anyhow).
Also the charger that Sofirn provides works but I wanted something better. My choice was the Miboxer C4 Upgrade. The Miboxer C2-4000 would fit a 21700 also.