Well as always, depending on your use case and your threat model, using those swap services might not be the best idea, specially if you want to be "stealthy" when purchasing or selling xmr.
If you buy or sell XMR in simpleswap and use a KYC'd coin (like BTC that you withdrew or deposited from/to a CEX like kraken or binance), then authorities can ask the simpleswap owners and they will show them that you traded Monero.
Again if this is not a concern to you, then perhaps those swapping services are "good enough", even though I have read countless stories of them performing shotgun scams and shotgun KYC where they will randomly select you and make it very difficult for you to recover your funds, especially if those funds do not come from "trusted" sources aka another CEX.
But again as always, that is up to you and your use case / threat model, and the risks you are willing to take.
I would like to add that if you actually have XMR in a wallet that you got from mining, and you never moved those funds before, you CAN confirm that those funds came from mining. In fact, the GUI wallet will show a symbol indicating that it came from mining (what's called a "coinbase" transaction).
However you could just say that you moved the funds from the mining wallet to this other wallet, but then I will ask again: Show me the mining wallet that got those mining transactions.