Using Muun is your first problem - they do submarine swaps and isn’t real lightning so fees are astronomical. You’ll always be exposed to on chain fees while on chain. You have to run a node and open lighting channels from your on-chain balance. Once on lightning, those channels theoretically can be open forever. If you’re using custodial wallets for now then I would swap to Wallet of Satoshi. Some people will recommend Phoenix too but I haven’t played with it yet. Blitz wallet is a way to run a node on your phone. Otherwise check out plebnet.org for node running.