Are there any other services similar to this
You can do it with anyone willing to pay for your transaction. The Bitcoin protocol support transactions with multiple inputs and outputs. If you want to test it, you can use any test network like testnet, regtest or signet, and create some sample transactions. Later you can repeat the same with real coins, when you will know how it works in practice.
With Bitcoin Core you can create any transaction you want, sign it with that wallet, and broadcast it directly (if you have the whole chain) or copy hex data and broadcast it by using some online tool. Creating and signing can be done offline if you have all transaction data, if you know all inputs, outputs and previous transaction scripts and amounts. Probably other wallets like Electrum also support that kind of things.
As long as you can find someone willing to mix some coins with you, there should be no problem with that, just share all inputs and outputs with that person, prepare transaction by including all of them, sign it and broadcast.