This solution doesn’t have to be crypto at all, however. I’ve been to China where even food carts have QR codes printed on them for people to take wechat pay and ali pay.
We have Bank QR codes and something simillar to M-Pesa, but its not that widespread because there is no one standard for QR payments or similar. That makes it less practical to use because both you and the vendor must use the same bank. (El Salvador is small and there are many finatial instituions.)
Something like U.S. Zelle or Swedish Swish would help, but banks have not standardized yet.
If WhatsApp started something like WeChat payments it would become a hit. Like they did in Brazil.
Free mobile banking accounts have been available for a while in El Salvador, and can be opened with a selfie and a photo of your national ID. They are only avaialble for those 18+ and older though.