Just finished PS4 trial... Question!

[Eltaire]

Or should I wait to purchase when the two will likely be bundled together?

You'd likely be waiting a year or so for the game to be bundled together.

What I'd suggest is buy the base game now, and look at purchasing the expansion only after you have gotten through all of the A Realm Reborn content. There's quite a lot of it there, so you're not likely to burn through it all quickly unless you're a hardcore player.