Nice one Mark. Always nice to see people trying new natives.
If it was me, I'd repot and separate the roots now. I would try to slowly ease apart each root mess and spread them out early, (mainly having the hose on a weak stream and slowly separate the old soil and separate the roots without cutting off anything if you can manage it.
But if you leave them for a year or so, you then have the same tangled material which has then got even more tight and compacted.
In my opinion slip-potting is a last resort and kind of counter-productive, especially if the material is healthy.
This way you know in a month or so if they have survived, rather than finding out later on that years of effort and maintenance has been for nothing.
It is hard to properly maintain and water a lot of trees, so in my opinion its better to know sooner which ones are going to survive and give those better care and attention.
