Authenticating Solidity params and signatures within smart contract - solidity

Question

I'm trying to build a sort-of meta-transaction architecture where an admin can trustlessly submit someone else's transaction with validated data. Basic Solidity and JS code below. I'm missing a piece somewhere as the signature verification fails, but not sure where
...
Basic contract version:
contract Account {
struct User {
string username;
}
function createAccount(
address _signer,
User _user,
uint256 _nonce,
bytes memory _signature
) {
bytes32 messageHash = keccak256(abi.encode(_user, _nonce));
bytes32 signedHash = keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", _messageHash));
require(recoverSigner(signedHash, _signature) == _signer);
...
}
function recoverSigner(
bytes32 _ethSignedMessageHash,
bytes memory _signature
) public pure returns (address) {
(bytes32 r, bytes32 s, uint8 v) = splitSignature(_signature);
return ecrecover(_ethSignedMessageHash, v, r, s);
}
function splitSignature(bytes memory sig)
public
pure
returns (
bytes32 r,
bytes32 s,
uint8 v
)
{
require(sig.length == 65, "invalid signature length");
assembly {
/*
First 32 bytes stores the length of the signature
add(sig, 32) = pointer of sig + 32
effectively, skips first 32 bytes of signature
mload(p) loads next 32 bytes starting at the memory address p into memory
*/
// first 32 bytes, after the length prefix
r := mload(add(sig, 32))
// second 32 bytes
s := mload(add(sig, 64))
// final byte (first byte of the next 32 bytes)
v := byte(0, mload(add(sig, 96)))
}
// implicitly return (r, s, v)
}
}
Basic JS script:
const userEncoded = ethers.utils.AbiCoder.prototype.encode(
['string'],
['abc']
);
const nonce = 1;
const hashedMessage = ethers.utils.keccak256(userEncoded, nonce);
const signedMessage = await user1.signMessage(hashedMessage);
await account.connect(admin).createAccount(user1.address, { username: 'abc' }), nonce, signedMessage);