I would try Ardor. It's not finished but already tested and offers the first decentralized sharding solution for cryptocurrency scalability that is not based on centralized sidechains.

ELI5: With Ardor, full nodes only need to download about 1/100 of the blockchain data that are required with Bitcoin-based cryptocurrencies, given the same transaction capacity. The tradeoff is extremely small (clients don't validate all old transactions, only important ones, but new TX are validated properly).