@Flox, first of all, good luck 
based on the thread earlier in the year where I replaced all of my drives, @phillxnet recommended to add one disk first (if I had a port to spare), so that there is enough room for balancing activities. As I was on RAID5/6 that was more important than for other RAID configurations.
You mentioned early in this post that you’re pool is pretty full (2.7TB), so I am wondering whether it would pose a risk if you run out of space during balancing as well. Here’s the caveat that I got from Phil back then
It sounds to me that you would be better off with Option B or C. I essentially did option B for my “all drive” replacement because I didn’t want to fiddle with the “replace” option, but I also had plenty of time (but also bigger drives to replace). At that time (and as you can see from that thread I quoted from) I was already on a “bleeding edge” kernel as well as btrfs tools … but still didn’t trust the replace option (not out of any experience or major testimonials, it just sounded iffy especially for RAID5/6, which you’re not using).