1. I read that never store all your coins in one wallet. So wts the solution? should I install two electrum standalone wallets and store my coins in both of them?
You should have 2 wallets because if you lose a device or it gets burned you can still recover your coins. Having two wallets which are exactly the same in the same device defeats the whole purpose of having 2 wallets.
You should have at least one for big amounts which you will never touch (like a hardware wallet) and one other for small transactions (a mobile for example)
2. I have written seed on a paper to recover my BTC in case my computer got corrupt. What thing apart from seed should I backup to recover my wallet in case my system is corrupted.
Nothing else.If you made a passphrase, you have to note it down as well, but it is almost part of the seed.
3. Strong password is good enough to secure a wallet or I have to do something else also.
No, a password in your wallet doens't give you any additional security, only additional privacy (as nobody could look in your funds without it). but your funds can be retrieved without the passwrod, if the hacker gets the seed.