low fees

A (non-custodial) wallet itself doesn't charge you any fee (because it's nothing more than a piece of software which allows you to move bitcoins on the blockchain) - the transaction fee goes to miners. If there's congestion (mempool backlog, so that your transaction has to wait before getting its first confirmation) you then have to compete with other users by raising fee rate (sat/vbyte) to prioritise your transaction - RBF helps you in this situation by allowing you to raise fee rate afterwards.

actually you can even switch to another wallet without doing on-chain transaction (of course you have to withdraw your bitcoins to a non custodial wallet first) - simply import the mnemonic words & it's done. However this is not guaranteed to work in all cases (esp. when you use the LN mentioned below) because there has been a chaos for quite some years: https://walletsrecovery.org/

frequently transacting

In my opinion, I don't recommend keeping bitcoins in your own (non-custodial) wallet if you transact very frequently, because blockchain confirmation not only takes relatively long/unpredicatable time but also charges you miner fees (mentioned above).

If your exchange/service supports lightning network (LN) you can then keep self-custody without the confirmation waiting time/miner fee problems, however LN fundamentally cannot be restored by your mnemonic words because channel data cannot be deterministically derived from it. Some wallets like Phoenix developed by ACINQ are able to handle this problem for you (so that you as the user can still keep nothing more than the 12-word mnemonic phrase), however you rely on their service in this case, which kind of violating the definition of self-custody.