How to quickly get the target of the next block to be mined from bitcoin-core over RPC?
You can easly calculate that from the difficulty
So just with the RPC call getdifficulty calculate this:
target = targetmax / difficulty

Where targetmax = 26959535291011309493156476344723991336010898738574164086137773096960
target max is a fix number from the genesis block
If the hash value (Converted to decimal) is lower then this value, it's a valid Block