In sectioning strut tubes, it seems it is common practice to leave the insert in while welding, taking care not to weld the actual insert to the strut housing.
If you purposefully leave a gap, then you are more likely to accidentally weld the insert in.
The inner sleeve method would be the best by far, and if your insert fits in with this in place, go for it, just make sure you don’t rely on thin wall tubing for your strut housing and make sure it is ALL weld.