In your case, you have drives with different performance characteristics. I wouldn't combine them at the btrfs level, I would combine them at the filesystem level.
In other words, create a btrfs partition on the new drive completely separate from your other btrfs partition. Then create a subvolume for steam. Next add an entry in /etc/fstab to mount the steam subvolume to where you want it to live in your filesystem.
That way your nvme drive and you sata ssd will be separate but still accessible from within the same filesystem.