You misses the point. It does not matter what kind of technical implementation or algorithm is used. If the average user hardware can solve the "captcha" in a meaningful time on average hardware then an attacker with optimized hardware and on scale can always solve millions of these "captcha" relatively cheap. If you increase hardware demand to slow down an attacker you just exclude more and more legitimate people. Sure the attacker maybe can only spam 500k messages instead of 1MM in the same time but you also reduced the legitimate user by 50%.

Even in the absolute worst case where no optimization is possible at all the attacker can still run a device 24/7 so if a normal user has to wait 20 second on a smartphone an attacker can spam at least 4320 messages per day with the same device. And it scale at least perfectly linear. 2 such devices would double the spam capacity.Aand if the block sizes are increased to slow the attacker down it is exactly as much as it slows down the real user. But the real user actually cares and gets annoyed the attacker does not, he keep the same spam/legit message ratio.

Not sure if that is joke but Bitcoin mining is already highly optimized and done on scale.

Beating the average smartphones in-browser hash power does absolutely nothing. You are not competing against them you compete against large scale mining farms with special hardware.