how about cutting the difficulty in half instead of dropping it to 1 after 20 minutes? That will quickly solve the problem if the difficulty is too high, and a large miner stops. But it also disables CPU mining: if the difficulty drops by a factor 8 in an hour without blocks, it takes many hours to reach 1.
Difficulty doesn't just drop to 1 if no block is found for 20 minutes, someone has to find that block (20 minutes later) or otherwise find it sooner and (lie about it), Difficulty in the last few days was 500M, this means a few years for a high-end CPU to solve it, heck, it's actually even difficult for your average ASIC gear to solve a 500M block, an S19 pro would need about 6 hours to solve it, there must have been close to a petahash hammering testnet4 for that 500m to be sustained.
So, back to your proposal, It would take roughly 9 hours until the difficulty drops to 1, and mind you, there must be a single honest ASIC who would not abuse the 20 mins rule and won't mine his own blockchain, that honest guy would mine the 1/2 difficulty block and disappear for 20 mins, come back, hit a block, go offline, do this for 29 times just so that the broke folks can mine on testnet4, we all know nothing like that is going to happen.
Furthermore, your solutions lines perfectly with the proposed fixes of elimination the difficulty 1 all together, because here is the unsolvable issue related to testnet.
If difficulty doesn't reset = CPU will never hit a block.
If difficulty is always 1 during the entire epoch = thousands of blockchains will be created and constant block reorgs, all nodes will be rekt.
If difficulty keeps growing forever = even small ASICs won't be able to hit a block.
It's like a race where you would assume that everyone would be running on their own feet, then somebody bought a horse and started beating everyone else, you adjust the rules so that if the horse leaves, the benchmark now drops, the same horse leaves then comes back to break his "own" benchmark even more easily, you say you want to keep a high benchmark, you now just forced the horse to run faster and made sure not a single runner would beat him and that now everybody must buy a horse.
The next thing you know is, people start racing using cars, when that is fixed, they use faster cars, and the story is never going to finish.
There is really one possible fix to testnet and that would be a consensus of resting the entire blockchain at block x, x should not be anything more than 1,000, with a "currency" that resets every 2 hours, nobody would want to hoard it.