That means it would cost ~$0.025 cents (300 sats each) to post anything on memo.cash. If it went to $80k, that means it would cost $0.25 for each post. 1/sat per byte is already minimum.

Correct, but the minimum fee would be lowered before that, it is just a question of the default forward limit in most nodes, which can be changed no problem. Wxample 10 sat per 1000 byte