In an ideal world, the OP will be able to replace the SMR drives with CMR drives on the basis of goodwill from WD. That's the first thing I'd try and thank you
@FJep for suggesting that step. Chances are, the replacement drives will be refurbished, but so are the SMR drives being sent back in. Hopefully, there will be some sort of warranty. Additionally, I'd build the CMR-only pool with Z2 VDEV(s) unless there are multiple backups nearby.
As for the gradual replacement of SMR drives via CMR drives, the OP has mentioned that the SMR drives work fine on a day-to-day basis. To me, that signals that the OP's use case / workload is light enough not to repeatedly trigger known DMSMR issues such as CMR-cache transfer, garbage collection, and so on. Thus, I still consider the gradual replacement of SMR drives with CMR drives an option since the pool use appears to be light. But yes, I'd leave that pool alone while it resilvers (I do that for my regular, CMR-based pool also).
On higher-workload machines, the impact of SMR drives becomes very apparent once the drives start filling up, stuff gets fragmented due to heavy work, and the SMR controller never gets the quiet time to defragment, flush the cache, collect garbage, whatever. That's how Patrick explained the time delay between WD starting to ship allegedly-qualified DMSMR NAS drives and customers starting to complain to their vendors./ forums re: cratering pool performance.
Customers like Backblaze would catch this stuff quickly (they have foresworn the use of SMR drives altogether) but they likely do not buy sub-8TB drives at this point since they're transitioning to 12TB drives and beyond. Thus, there was no quick market pushback from high-volume buyers to alert the management group at WD that the DMSMR-for-NAS-drives decision was a really bad one. Like Patrick Kennedy, I would like to give WD the benefit of the doubt re: boneheaded business decisions.
However, the more cynical among us would infer that WD knew this thanks to presentations given in 2015 re: the unsuitability of DMSMR for NAS applications and hence
- only saddled the lower-capacity NAS Reds with DMSMR technology
- did not update their spec sheets to reflect this very important change
- took longer than they should have to acknowledge the issue
More importantly, I wish that WD simply bit the bullet and offered all its customers with SMR Reds the option to replace them with refurbished / new CMR Reds without a lot of fuss. That is, set up a web site, let the buyers enter the relevant S/Ns, collect a CC number for cross-shipment, and presto, some customer goodwill is restored. The current drip-drip-drip approach to press releases screams that there is tumult @ WD re: who is going to be held responsible.