TLDR; For WHM healing: Det > Piety >= Spell Speed > Crit > Acc For WHM DPS: Acc > Det > Crit > Spell Speed >= Piety

It really depends on the accuracy cap of the content you will be doing and whether you plan on DPSing or being strictly a healer.

Most people who do high level melds target the accuracy cap for A8S since that's the highest accuracy requirement.

That would be for DPS though. For healing, there are no accuracy requirements. You will never miss a Cure spell.

Healer stat weights are hard to come by since their primary function is healing, not DPS, and depending on how you prioritize those two functions will change your needs.

For example Crit is very important for DPS because it scales with all other damage modifiers, but that's moot if you don't have enough Accuracy to avoid missing. Crit is also important to SCH for Adloquium.

For WHM healing purposes however, Crit isn't as desirable because even though it does boost your healing it's a boost you can't rely on getting, and therefore you'd mostly need to heal as though you won't get the crit boost which makes it a bit of a waste and leads to more overhealing when you do get the crits.

Spell Speed is frowned upon by a lot of folks, but it can mean the difference between getting a cast completed vs. being interrupted while dodging mechanics (and all stats have zero value if you don't complete your cast). So that's a sort of immeasurable value that I really like.

Generally speaking DET is always your best choice for a secondary stat as it provides reliable healing potency being a weaker version of whatever your primary stat is.

Piety is also important, but not as important as DET. The stats sort of all play on each other. Piety gives you mana, but DET saves you mana by being more potent. Spell Speed enables you to use up mana more quickly, but just because you have it doesn't mean you want to burn as much as possible.