The Honda NX500 is a superb machine in nearly all respects when considering it through the lens of a prospective ADV rider - except for its steep price tag, especially after the recent GST 2.0 reform. Nevertheless, this is a machine that is likely to last you a long time without any significant hassles and will also hold its value well should you decide to sell it someday in the future. Coming to the Royal Enfield Himalayan 750, there are absolutely no concrete details available about the bike currently, so commenting on its reliability or after-sales service isn't possible.
Even if you do decide to pull the trigger as soon as the big Himalayan is launched, we would strongly advise you to wait for a minimum of 6 months so that if there are any teething issues, they are solved by the time you put your own money down.