ERC20 token contract is.. well... the contract which is the token. A token is always a contract. Crowdsale is a contract which is used to sell a token

Hmm, what are my options for sending a transaction to a contract when I don't have the full source code nor the ABI, but I know the function signature I want to call?

you can work on the byte-level operation

in my experience, the tx data consists of function signature || data

https://medium.com/@blockchain101/calling-the-function-of-another-contract-in-solidity-f9edfa921f4c

Yeah that's what I'm doing currently.. But it's just.. horrible. Like, isn't there at least a tool somewhere to convert my input into the required hexdata?

Is there any tutorial out there for integrating erc20 token contract with crowdsale contract?

Probably, no idea which would be up to date. Just try searching for one - make sure it's from this year

Crowdsale contract has been removed from open zeppelin since v3.0. and i didnt found any latest tutorial on that.
Can anyone help from this group for integrating erc20 oz contract with oz crowdsale contract?

Do you mean stuff like SafeMath? To prevent integer under/overflows? or what do you mean with security

The node alone will take around 4TB storage

Ah yes infura, too