Yeah, I think v18 was future tense at that time, but has already happened. Oh yes, the whole earth has been filled with the knowledge of Him already; when He told His 12 apostles to go into all the world and preach the kingdom to every creature.
I believe things as we know it are only temporary. Also, that whole thing about a new heaven/ earth is an enigma; we’re really not given much to go on other than it will be beautiful!
True, except that the Twelve failed to do much, but Paul picked up their slack, although even now much of the world is ignorant
of the Gospel, thanks mainly to Marxism and Islam.
I think we are not given much to go on regarding the description of heaven because it is impossible for us to imagine alternate reality,
which is why Isaiah described it in earthly terms.