there's a big difference between a passphrase and an extension word...
In case of a password/passphrase/pin => If you have the same seedphrase and use it on different wallets and use a different passphrase (or pin, or password), you're undermining your security. If one of those wallets is vulnerable and the attacker is able to get his hands on either the seed or the mpk, the attacker is able to rob all 5 wallets since the password merely encrypts the mpk, so it doesn't matter if you used different passwords to encrypt said mpk (since he'll steal the unencrypted version anyways... Or the encrypted version which only needs to be bruteforced once, not 5 times).
If you're extending your seed with a 13th or 25th word, things are a bit different... This being said, if for some reason an attacker exploits a weakness in the wallet that allows him to capture the first 12 or 24 (or whatever number) of seedwords, he only has to bruteforce this extension word 5 times, which is far easyer than bruteforcing the complete seed + extension word (which is impossible). Offcourse, a long extension word makes this a lot harder (if not practically impossible). This is basically my setup, but i keep no unencrypted version of my seedphrase and i only use hardware wallets to store my funds.
For your next question: keeping the password or the extension word in an online password manager decreases your security... If an attacker is able to exploit an attack vector that lets him get his hands on your seed phrase he no longer needs your password. Keeping an extension word in an online password manager will require him to steal your seed + brute force his way into your password manager, which is hard (but certainly not as safe as keeping everything offline).
Basically, the "ideal" way to create the wallets is completely offline seed creation + completely offline extension word. The best way to store the seeds are 5 different seeds + 5 different extension words saved in at least 2 safe places, and never store seed + extension word @ the same place... All other things described in your post decrease your security.. This being said: you might be fine willing to decrease your secutiy in order to increase your redundancy of backups, but that's very hard for a thirth party to decide... You're probably fine re-using the seed and adding a very long extension word that you keep in a very secure cloud environment using a very hard passphrase for encryption, but I would never do this since for me it wouldn't feel secure enough (but maybe for you it does?).
Personally, i have one 24 word seed phrase + several extension words. I use this seed on my 2 hardware wallets, and i keep said 2 hardware wallets in two safe places. I have different wallets on both hardware wallets by using the different extension words. I then used ssss to split the seed up into 3 parts using a 2 out of 3 scheme with passphrase encryption and i stored the 3 slices in 3 very safe places. I did not keep several copy's of the seed phrase, since i have 2 physical wallets + one encrypted copy of the seed phrase split in 3 parts using a 2/3 ssss scheme. Odds of me losing both hardware devices and 2 out of 3 slices are negligible (since the storage spots are physically far apart... It would basically need an atomic bomb nuking half my country in order for me to lose access to my wallets).
If an attacker:
- gets his hands on one slice: he can't do anything since he needs 2
- gets his hands on two slices: he needs to bruteforce the passphrase of the ssss scheme + the extension words
- gets his hands on a physical hardware wallet and bruteforce the pin + the extension words
The thing does remain: there are always attack vectors... The more attack vectors you eliminate, the bigger the odds of you losing access to your wallet or funds... If you try to make up schemes to make sure you will never lose access to your funds, you'll inevitably open up very small attack vectors for potential thiefs. It's very hard to find a balance.
