It’s little bit stupid you will likely not be able to withdraw everything 100%, some sats will remain. Ok, not that it will change my life to donate some sats for other free software devs, but, anyway, not the best use experience. They could have implemented “withdraw everything to onchain” and just take some 1% as a swap fee.
Discussion
Just noticed - it looks Phoenix wallet have such feature, called “drain wallet”, where you enter onchain Bitcoin address.
Although that could be done using LNURL or BOLT12 over Lightning too, just try amounts one by one, reducing by 1sat each time until your withdraw amount is total wallet balance minus fees.