This fee is basically a mining fee, because in Phoenix you actually have your own Lightning channel, which takes an on-chain transaction to open the first time. Later on, with this channel already opened, the fee will only be that of a Lightning transfer, so presumably a few sats.

See this table for more details (in the "Fees after" column, since you're running the latest version of Phoenix).

As you can see, you're in the "receiving Lightning with insufficient liquidity" case, i.e. the last line of the table. The fee is actually smaller than what you would have had to pay with the previous version of Phoenix, which didn't have splicing. For more details as to what 'splicing' means in that context, you can take a look at the "Wallet & Tools" section of this newsletter I wrote, or to Phoenix's blog post.

Using Lightning with full control over your funds (i.e. "self-custody") comes at the unescapable cost of having to open a channel. Wallets that don't allow you to have your own channels and private keys are custodial: you trust them not to steal your funds, like a bank.

So you're doing nothing wrong. I would even say, you're doing it right! ⚡️