To me, a satisfactory solution to metaethics might be an algorithm for computing morality that can be isolated from the rest of a human mind

The problem is finding this algorithm. After you find it, you may isolate it from the human mind.

It's like if humans would instinctively calculate 2+2, but we wouldn't be aware of what exactly are we doing. So we would need some way to discover that we actually calculate 2+2. Later, when this fact is known and verified, we can make machines that calculate 2+2 without having to inspect human mind.

along with some explanation of why this algorithm can be said to compute morality

Such explanation would include comparing with a human mind. You can explain that the machine calculates 2+2. But to explain that the machine does the same thing that humans instinctively do, you need to compare it with a human mind.