What do you mean by "Grin’s privacy depends on network activity" ? Grin's hiding of amount and address doesn't depend on that. Only the use of CoinSwap to hide the tx graph would depend on a sufficient amount of daily CoinSwap txs.
While Monero's privacy is a little better, it pays a large price for it: having a 20x faster growing chain, and the need to remember randomly accessible data from every output ever spent.