Not sure if testnet could be indicative of what happens on mainnet (security budget wise). Because there is no financial incentive on testnet, miner game theory is quite different between those two.

Just thinking out loud:

Would a testnet without 20 minute rule work, now we have a lot of 'hobby' miners also? Maybe not, because difficulty stays too low, and an adversarial miner with enough hashpower can cause huge reorgs.

What if the difficulty adjustment would not take into account difficulty 1 blocks, and increases/decreases relative to the previous difficulty. I think this makes 'fair' mining using an asic easier, but blocks will flow in much faster than 10 minutes.