I've seen lekker rendered as both tasty and delicious. In English, "tasty" is OK, pretty good, nice, but "delicious" is much more intense, superb, fantastic, great. Does lekker cover both ranges in Dutch? How can I congratulate a host on a really excellent meal? Will lekker suffice?
if you really want to give a compliment you could use 'heerlijk' (delicious), instead of 'lekker' (tasty). the foremost meaning is regarding food, but the use of these words is much wider.
It looks like lekker is also synonymous with good.
Is the term lekker typically only used when referencing a food item? You would not reply to "❤❤❤ gaat het?" with "Lekker!", right? (You might... but that might be less than appropriate..?)
You could, but it can have a different meaning. If you enjoy what you're doing, you could say 'het gaat lekker', however it can also be sarcastic: 'het gaat weer lekker', although the literal translation would be positive, it is commonly used to mean something like 'it's all gone sour again'.