How to create Tangent Complex Curves

I'm trying to create a single shape - but one with complex curves:
A 'rounded heart' shape.
I believe the construction should be relatively straightforward, as the object is symmetrical front/back and left/right

I believe the intended way to create such an object is:
* Sketch the silhouette of 1/4th of the object from front, top, and side
* Use loft + guide to form a surface defining this 1/4th
* Use mirror/fill to flesh out the rest of the symmetrical object

The sketches that describe the silhouettes all use splines to try to enforce tangency by the mirror planes, while also constructing the smooth curves

However, no matter what I try, I cannot seem to get tangency between the faces.
(Tangency is desirable, as I am later going to be performing complex intersection/split/thicken operations on the faces, and thus want them to be a 'single continuous curved surface' and not 'four separate faces')

Some things I've tried:

• Adding straight portions where the sketches meet the mirror planes to make tangency more obvious
• Creating surfaces that extend out from the curve, then lofting tangent based on those
  (a la this post)
• Filleting and face-blending these unwanted edges
• Trying to use some combination of replace-face to merge the faces
• Using fill with tangency instead of loft

(Although several posts discuss similar topics, none of the solutions seem to apply here - but feel free to direct me if this is solved elsewhere already!)

What am I missing here? Do I misunderstand something about tangency? I'll be honest - I can flick on all the curve diagnostic tools, but I fail to see what how they would direct me to a solution here.

Check out my example heart here

Thank you for any help or advice!