Hardware wallets like Ledger Nano S and X, Trezor is safest place to keep altcoins. All other wallets (desktop,mobile, web and others) is significantly less secure. Apart from hardware wallets, Coinomi is good option IMO.
Coinomi is closed-source, and I would almost never recommend the usage of closed-source wallets. Exodus has both a desktop and mobile app, so I'd suggest using that instead. Though I'd never recommend holding a huge variety of coins in the first place for obvious(smelly) reasons.
You have valid point, but I'm not sure why you're suggesting Exodus, when they aren't open source, same like Coinomi.
And Ledger wallet isn't open source too, but it doesn't makes them less secure to use.
Coinbase is a wallet that has been operating for a long time and has a good reputation, but the type of altcoin that can be stored on coinbase is not very complete, but to save coins that are already popular I think coinbase is good enough, I think most people will exchange their coins for popular coins to store.
Coinbase is least secure place to keep your coins, same like all other exchanges. There are many reasons why. You don't have full control of your coins, it's centralized and requires KYC and it can be hacked. Exchanges shouldn't be used as a wallet.