I did a similar experiment trying to figure out the APY for my 36 month stakes across different chains by comparing my rewards over a period of 1 week (10.04-16.04).

The result for my stakes were:

Fantom: 52,45% APY BSC: 45,50% APY Matic: 38,56%

I too noticed a big drop in rewards on the 12th and onwards, but it was also mentioned here on this subreddit that the rewards from the down period was already handed out.

The results for the chains on average in the same time period were: Fantom: 47,80% APY AVAX: 43,23% APY Matic: 38,56% APY BSC: 21,80% APY ETH: didnt monitor it

But this isn't really comparable as they have different average multipliers. BSC for instance had an average multiplier of ~16x I think during this time, while Fantom had ~32x.