We know from theism that the Divine is the source of all being (indeed Being itself), is also that from which every thing else proceeds. However, this does not imply that He is All There Is, just that he continually sustains the being of all creatures derived from him. This is necessary for the Lord loving us, since he cannot love himself:
"It is the essential of love not to love self, but to love others, and to be conjoined with others by love. " and "Love consists in this, that its own should be another's; to feel the joy of another as joy in oneself, that is loving.But to feel one's own joy in another and not the other's joy in oneself is not loving; for this is loving self, while the former is loving the neighbour. These two kinds of love are diametrically opposed ..", as Swedenborg reminds us in DLW 47.
It would appear that if the Lord is all there is, then when He loves us, He is loving part of Himself. Don't you see that this is the love which He cannot have? There must be something in creation which is distinct from the Lord, although created and sustained by Him.
We read of the many (wonderful!) experiences of unity of the One Life that all mystics (and many others!) have had over the millenia. That is the way the world is!
I suspect that the desire to say that the Lord is 'all there is', comes from wanting to interpret the experiences of the One Life as 'non-dualism', following the advaita approach. You also want unity with that approach too.
However, a common meaning of 'dual' (which should be opposed) is one of a very fragmented world. We all know that this cannot be true! (i.e. should know!). Therefore often we need to replace everywhere 'non-dual' by 'non-fragmented', and 'dual' by 'fragmented'. This conveys the common meaning much more precisely, as often the explanation of 'duality' is fragmentation.
Then, we are able to find that in creation which is distinct from the Lord, which (whom) he can love without being selfish, and in whose joy he can delight. All of the wonderful mystical words describe these delights, and it would be a great shame for any presentation, if 'non-dual' were taken as the 'non-Distinct' nature of the Life from creation! The One Life is the lifeof creation, but is not equal to creation.
This is the reason for there being a creation, that all the delights of spiritual live and love become possible! This requires a supreme doctrine of 'non-fragmentation' (sometimes confusingly called 'non-dualism'), along with an equally-supreme doctrine of 'Distinctness': what we call Discrete Degrees.
More details in the book at Starting Science From God, especially chapter 9.