I was wondering if there is a way to use a custom seed in Bitcoin Core v23, rather than allow the program to use the default method to try to get entropy.
Yes. Use the
sethdseed[/itt] command. Link: https://bitcoincore.org/en/doc/25.0.0/rpc/wallet/sethdseed/
Your custom seed has to be in the format of a WIF private key, so if you are using some manual process to generate the entropy, you'll need to convert it to WIF first. Entropy -> 32 byte hex -> prefix network byte -> calculate and append checksum -> encode to Base58.
The simplest way to use a physical process to generate a private key will be to flip a coin to achieve a 256 bit number, and then convert that hex.