By 51% attack, I'm specifically referring to an attack in which an adversary with more than half the hash power of the network is able to double spend. No matter how high the hashrate is, the risk of a 51% is always there.

This turn-based mining eliminates the risk of a 51% attack because hash power no longer influences a miner's win rate. It introduces competition when necessary to deal with slow or non-responsive miners and employs POS to prevent spam. Here's the idea:

• Every block contains a list of miner payment addresses.
• Addresses are added to the bottom of the list from an address pool, similar to the mempool.
• When a miner's address reaches the top of the list, they get the opportunity to mine a new block.

When a node wants to mine, they must stake their address to get it added to the pool. When the next block is mined, the miner modifies the list from the last block by including any new addresses from the pool to the bottom of the list and removing their address, plus any addresses above theirs, from the top of the list.

If a miner takes too long, more than a single block interval, the second address is free to compete with the first address in the next block interval. Each block interval is a turn. If they both fail to produce a block in the second turn, the third address on the list is free to compete in the third turn. The pattern continues until a block is found. When a block is found, the next turn goes to the address after the one who produced the last block. For example, if the second address finds a block in the third turn, the next turn goes to the third address.

Turn based mining helps decentralize mining by eliminating the incentives for pool mining. Pool staking will be a thing though, increasing demand for the coin. Mining difficulty is still determined by the block rate, but the block rate is more consistent, and the difficulty will normalize to average solo mining speed. It also saves a lot of energy and improves efficiency.