Could you explain how the exchange mechanism works ?
Nicehash is efficient paying in BTC because there is no exchange involved. Other auto switchers fail because the long delay between profitability calculation/mining and exchange to BTC or other target currency.

What is the delay here ?

One useful feature that could mitigate the problem with rapid price fluctuations and exchange delay is to have the auto switching weighted towards difficulty priority instead of price priority.
If the profitability calculation makes a coin raise to the top because of a rapid swing in price, it may be worth less compared to if the difficulty drops for that coin. These two parameters are connected due to all bots and auto switching running out there so it's not easy to say what would work.
However, with some development effort, you could deal with this. You need to systematically collect statistics for a number of coins over time and save to a database. Then you can simulate different profitability strategies on that data and see what happens.

If you are really ambitious, you can use such data and feed it into an AI model. Then, the system could learn the market patterns for different coins and anticipate which coins will be most profitable to mine and when to send them to exchange. This may be the only way to beat Nicehash by a significant margin.

Having said all this, I thinks it's really good that you give Nicehash some competition.