that is why I asked if I can set-up an airgapped wallet using an old laptop. Unfortunately, I realized that it is a very tedious work and there is a high risk of losing money involve since I do not have any idea in setting up a Raspeberry Pi.[/quote
You
can set up an airgapped wallet using an old laptop. It doesn't have to be a Raspberry Pi - this is simply the cheapest option if you are buying a device specifically for this purpose. Any old laptop you have lying around can be repurposed in to cold storage.
However, I decided to just purchase Ledger Nano S as my main wallet in storing most of my crypto for long term hold. After that, I'll set up my current Android phone as the "watch-only wallet" and the other as "cold storage wallet". Is this is a good idea?
It depends what you are trying to achieve here. A Ledger Nano S is perfectly acceptable to use as long term storage. You don't also need a cold storage wallet on an old mobile phone, although you can set this up if you want. In your situation, I would probably use the Ledger Nano as a cold wallet to store the majority of your bitcoin which you keep at home, and a mobile wallet as a hot wallet which stores a small amount of "spending" bitcoin which you carry with you. You can then top up your mobile wallet from your cold storage as needed.