4) Instead of hardware wallet can I keep the separate phone to store my coins in wallet app like trustwallet and keep disconnecting from the internet unless i want to do transaction. Is this a better choice?
Well.. if the separate phone is factory reset except for the latest security updates and free of unnecessary apps and you don't use it for
anything except for your Bitcoin wallet then it's at least safer than storing the coins on a phone that you use on a daily basis. Given that you keep the wallet phone itself safe and the wallet backed up of course.
However that solution is nowhere near as safe as a hardware wallet or a fully airgapped device, for the reasons mentioned by ranochigo.