"Auto-reset by coinbase/message" is a consensus footgun. You'll create partitions the first time a miner accidentally (or maliciously) trips the trigger.

If you want clean slates without drama, I would advise to use tools we already have. Signet for predictable, restartable public nets. Rotating the -signetchallenge is a reset with zero PoW guessing and no ambiguity. Ship a couple public signets (stable, fast) and rotate on a schedule.

If you really want a new PoW testnet, do a straight Testnet4: new genesis, magic bytes, seeds, and nMinimumChainWork. Don't make resets automatic; flip via new releases. Keep mainnet-like params (10-min target, 2016-block retarget). Consider dropping the "special min difficulty" rule -- that's what made testnet3 weird.

For local throwaway work, regtest/signet-with-own-key beats any auto-reset logic.