Coinos charges 0.4% fee for BTC withdrawals (plus chain fee), or 0.1% lower than Boltz. A lifehack is to withdraw Liquid BTC instead, for a minimal chain fee, and swap to BTC at Boltz (0.1% fee) or swapmarket.github.io (0.09%). But for larger amounts Coinos may not have enough liquidity, so chaining two swaps LN->LBTC, then LBTC->BTC on SwapMarket is your best bet.