You may be able to create a basic bitcoin mixer by reading github codes and miscellaneous scripts but backend requires knowledge and experience. If your users use mixer to deposit them on exchanges, you have to beat blockchain analysis companies and return transactions to your users with aml score safety. It's not easy as it was in 2016, it's getting very hard these days, more experience and advanced techniques are required to bypass modern restrictions and analysis.
Add to this the problems of changing fees, as the current fee price is very high, and the cost of mixing coins before sending them to the recipient will be very high, and therefore the mixer fees will be higher than many people can bear. Unless you have a lot of funded bitcoins, a simple problem such as an increase in fees may lead you to stop.
Why do you need to convert bitcoins in monero and then convert from monero to bitcoin? You are paying transaction fees and converting fees which will take a lot of amount from your deposit.
One of the mixers that provides this service
https://mixero.io/mix_advancedit is a layer of privacy by XMR (Monero) bridge, in short, they convert Bitcoin to Monero, and since the transactions here are difficult to track, your Bitcoin has been mixed.