BcacheFS is interesting (I tried it!) but it didn’t end up being stable for me. An sVDEV combined with the regular HDD ZFS pool likely brings close to bcacheFS speeds to datasets for my use case - but without the dataloss issues I experienced using bcacheFS.
I suggest learning about sVDEVs, dataset recordsizes, and compression before considering any other filesystem because the three can help address the biggest stumbling blocks re: ZFS performance in many use cases. (shameless plug, but here we are)