There is definitely a reasonable argument for casting the limbs separately to simplify the mould making process and it's a common enough thing to do with complex models like figures.
The problem with using something like lego is that you are committing yourself to a needing a very precise fit and alignment and any issues like shrinkage or slight warping of the mould could land you with problems which wouldn't otherwise be noticeable.
My approach would be to make the pattern so that you get a reasonably positive fit, but allow a bit of room to remove material after it's cast to refine the fit and go for a mortice and tennon or peg type joint which you secure with adhesive.
Depending on how much room you have to work with you could either make the peg and hole as part of the model or drill holes and use a metal pin afterwards as part of the final assembly.
As part of your general research it's probably also worth looking at the way that commercial resin model kits are designed for assembly eg