You actually can have multiple separately managed btrfs partitions with subvolumes in both. However, in most(but not all) cases this doesn’t make a lot of sense as you lose the ability of btrfs to share the space between them. In order to give you more specific advice, we would need to know how much free space you have on each partition and how your partitions are laid out on the disk.
There is definitely a small raw performance hit when you compare ext4 to btrfs. In most cases, it won’t be noticeable for desktop use cases but every situation is different so I couldn’t guarantee it.
If you have the space, it is pretty easy from my perspective. What do you find non-trivial about it?
Like any major filesystem operation, it comes with risks and you should backup your data first. As these things go, I think it is pretty straightforward.
You definitely need the free space, yes.
Unless you have a large amount of data and/or slow disks, converting the system will probably take less time than reinstalling and reconfiguring an entire system. Especially if it a long running system with lots of config.
Of course, you could always move the partitions around and use rsync to move the filesystems if you prefer.
Interesting. Which features are missing in this scenario? I usually run btrfs defrag afterwards to compress all the data.