Either dig up an old iPod (flash-modding would be a good idea, as 300+GB iPods are rarer) and put Rockbox on it for FLAC support, or dig up an old phone or something and get a decent USB-C adapter/DAC.

That depends on your use case, I just did a simple zpool with no redundancy because I wanted maximum speed/capacity and all my data is backed up on an external HDD. If you need redundancy, I would look online for how to configure that and what the optimal setup is.

Actually, I assumed you just had the SSD, if you have more than 256gb of free space between those HDDs, you can go ahead and remove the SSD from your zpool right now (unless your bootloader is there, then you’ll have to make an EFI system partition on one of the HDDs and install a bootloader first)

You need to add the new drive to your existing pool because ZFS stores data across all drives by default, similar to a RAID0. Then you remove the old drive and ZFS will automatically copy the data off the failing drive onto the healthy one and allow you to remove the failing drive with no data loss.

Use an Ubuntu live USB, all recent versions of Ubuntu have ZFS drivers baked into the live environment. Then you should add your new SSD to the ZFS pool, and remove the old one from the ZFS pool. Your m.2 WiFi slot should be able to host the 2nd drive while you do this, but if not you can use an external USB housing for it, you’ll just have to make sure that the ZFS pool knows its UUID so that it knows it’s the same drive.