It really depends on what one really wants to do. If one doesn’t have the private keys, then they don’t really own the asset and instead have a ledger entry.
As for a fiat gateway, Cash App hands down for ease of use and quick verification for kyc. Then if one wants to just invest and not necessarily trade actively Coinswitch is a great altcoin gateway - quick, easy, low fees.
True. But if you don't trade you can't maximize profits. If one wants to be cautious but still trade, one can set up a certain comfortable amount (Say 5k) on the exchange, any excess profits at an end of the period will be send to a bitcoin cold wallet. If your balance is below 5k don't top up but try to trade it up to 5k.
Alternative method is to store 10-20% of your cryptos on exchange for trading while the rest in cold wallet. Rebalance perodically.