Bitcoin does not have to be literally legal tender to be a legal means of payment in a country - because in most democratic countries BTC is accepted as a means of payment without being regulated in any special way. What any country in the world can learn from El Salvador is that nothing should be imposed by force, and my opinion is that BTC should be left as an alternative, not as some kind of obligation.
Unfortunately, Bitcoin is a very inconvenient coin for payments due to the long time it takes to confirm a transaction. If an analogue of the system as it was implemented in El Salvador is implemented there, then I don’t see the point. It’s probably easier to make a crypto exchange and crypto debit cards, and tax these operations.
As a rule, on-chain transactions are not instant, but there is an LN that enables instant and super cheap transactions, and this is exactly what El Salvador used, so the problem was not in the speed or price of transactions, but in the fact that people simply did not accept Bitcoin as a better alternative to the US dollar, which is the official currency in that country.