there should be an option to choose seed manually instead random generation should be kept optional.
This is a terrible idea. You should
never choose words manually to create a seed phrase, as the seed phrase you create will have very low entropy and be very insecure. If you do not trust any piece of technology to generate a seed phrase randomly, then you should create your own entropy using 256 coin flips like I said above and encode that in to a seed phrase,
not just manually pick words you think are somehow "random".
If there is way to do so please share a link or wallets providing this type of facilities.
User Coding Enthusiast has made this tool to make generating a seed phrase from coin flips easier:
https://bitcointalk.org/index.php?topic=5373505.0However, once again, it requires you to trust the code he has written is not just delivering you a pre-generated seed phrase. If you cannot read the code for your wallet, then you probably won't be able to read the code for this either.
I would also caution that if you are planning to generate your own seed phrase using coin flips, you
really need to know what you are doing since there are many many more ways you can mess up, create an insecure seed phrase, accidentally expose your seed phrase, lock your coins in addresses you can't access, and so on. This is why the vast majority of people don't do this and simply use reputable software or hardware wallets.