Using $key^{th}$ multiple of $\pi$ as a one-time pad With a key $k$ of sufficient length, say 128 bits, is it possible to use $k^{th}$ multiple of $\pi$ as a one-time pad?
Please enlighten on the security as well as practical aspects (only if secure of course).
That is not practical since either you keep a sequence of the bits of the $\pi$ or regenerate them every time. You may need to store 64GB sequence if you want to encrypt such files.
Then you need to multiply it with the 128-bit number. That is not practical, consider multiplication of 64GB number with a random 128-bit number.

OTP needs true random bit sequences like flipping coins, $\pi$ is not since the sequence is there! It is known to everybody.

In modern Cryptography, we use the 128-bit random key with a PRF to produce a long enough secure key-stream to encrypt the message like ChaCha. This is the way we encrypt messages. They are very fast like 3GB/s with AES-NI.
No need to store or heavy calculation like you proposed.

It is not KPA secure yet CPA!. Given message and ciphertext, we can find the 128-bit key by dividing.

How the 128-bit key is transferred is not clear. The better way to use DHKE than a KDF than encrypt with xChaCha20-Poly1305 with this you will get forward secrecy - if you DELETE the key after decryption - integrity, authentication, and integrity where your scheme only may provide confidentiality.