Welcome to Eorzea!

The main advantage of buying (preordering) Endwalker now is to get the pre-order bonus which allows you early access to the expansion (a week before launch) as well as the bonus items for preordering, namely, the Menphina Earring which when equipped boosts the battle EXP you receive for each enemy defeated by 30% up to level 80, so it will be very useful for levelling (the other preorder bonus item is a minion of a FFIV character).

But other than that, the only other bonus for getting the next expansion now is if you want to create a male viera character (or change your existing character to one via a vial of Fantasia), which is being added to character creation options with EW.

Otherwise you can wait it out if you'd like til you reach the end of Shadowbringers before investing in it, it's really up to you!