I have a pool of two raidz2s of four disks each, one twice the size of the other; every so often I replace one set of disks with disks that are 4x the size (i.e. I started with 4x250gb + 4x500gb, after a few years I replaced the 250gb disks with 1tb disks, right now I have 4xt2b and 4x4tb - and I get to store half as much data as the total capacity, so 12tb at the moment). If a disk dies close to the time I was thinking of upgrading then I'll replace that disk with one of the "new" size (but can't use the extra space until I do the rest of the replacement).

It works pretty well - by the time I'm buying disks that are 4x as large I don't mind throwing the old disks away. I've definitely avoided data loss in scenarios where I'd previously lost data under linux md (which lacks checksums and handles disks with isolated UREs very poorly).