I don't see why you should hold back now really unless you're keen to save a few dollars and wait for a sale.
For me, I played until the last main quest before starting Hearts of Stone, completed that, and then started Blood & Wine, finishing that before then going on to end the main quest. The approaches of other players will vary depending on a number of factors and tastes.
There's a significant tonal shift in the setting of Toussaint in the visual atmosphere as well as the writing. The tournament side-quest was especially memorable for me and completely different from anything in the base game. It is by and large self-contained though and you'd likely be better prepared for the more challenging encounters if you elected to finish the main quest first and play B&W later.