Some comments from @sipa in #12702.

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Yes, I think `importmulti` is superior in every way, except:
* Slightly less convenient to use (it requires redundant information, as you have to explicitly state what you want to import, and then give the keys/... to enable that).
* ~Doesn't support P2SH-P2WPKH yet~. Added in #14454

The latter can be fixed by just adding support for that case. The former can be improved by adding 'default' cases (e.g. if you prove a WIF private key, it can permit omitting the address, ...).

throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, "Cannot set a new HD seed while still in Initial Block Download");
    }
    LOCK2(cs_main, pwallet->cs_wallet);
    
    if (!pwallet->IsHDEnabled()) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Cannot set a HD seed on a non-HD wallet. Start with -upgradewallet in order to upgrade a non-HD wallet to HD");
    }
    EnsureWalletIsUnlocked(pwallet);
    bool flush_key_pool = true;
    if (!request.params[0].isNull()) {
        flush_key_pool = request.params[0].get_bool();
    }
    CPubKey master_pub_key;
    if (request.params[1].isNull()) {
        master_pub_key = pwallet->GenerateNewSeed();
    } else {
        CKey key = DecodeSecret(request.params[1].get_str());
        if (!key.IsValid()) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key");
        }
        if (HaveKey(*pwallet, key)) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Already have this key (either as an HD seed or as a loose private key)");
        }
        master_pub_key = pwallet->DeriveNewSeed(key);
    }
    pwallet->SetHDSeed(master_pub_key);
    if (flush_key_pool) pwallet->NewKeyPool();
    return NullUniValue;
}
void AddKeypathToMap(const CWallet* pwallet, const CKeyID& keyID, std::map<CPubKey, KeyOriginInfo>& hd_keypaths)