As far as I understand it, you can never move the spawn chunks. You can move your re-spawn (sleeping in a bed etc) but that does not mean those become the spawn chunks. That never worked that way as far as I know.
Chunkloaders are a good solution, instead of guessing you now know the chunks are loaded![]()
In my experience with the flat map, the server spawn is actually quite far away from the island, and ends up quite annoying if you share the game for another player to join (fall to the void, anyone?), or die and don't have a bed, or the game can't "find" your bed.