At what point should I consider a cold wallet?

I have been accumulating crypto for a few years now and at this point I have multiple thousands of dollars in my coinbase account. I store all my assets on coinbase and I am fully aware of “not your keys not your coins”. Hence why I am asking about this. The only cold wallet I know of is the ledger, but I have heard mixed reviews of them. Does anyone have a recommendation of a good cold wallet?

I am also unsure of how cold wallets work, do I need to transfer the funds back to an exchange when I am ready to sell? Do any cold wallets allow me to sell directly from the wallet? I know some cold wallets have apps that allow you to track your account value.

Also do I need different cold wallets for different networks? I have eth and polygon coins, can I store them in the same cold wallet?

Sorry if these questions are basic, I never expected to amass the value of crypto that I have.

Thanks