It's nothing to do with speed or planning etc. Whilst those are all factors in the overall thing the main issue is the size of the GW brand and how much is made in house.
Of course they're doing things like balancing safe working practices at the factory with ever increasing demand for models. GW took a stab at what they thought The Old World would sell, they turned out to be wildly wrong, so they have to reallocate production time to restocking that, in the mean time 10 other kits go out of stock, so they have to then work out what priority there is to allocate production time there is to restocking, and this will be done based on demand and complexity.
That production time restocking will include putting in orders for new boxes, instructions and books which aren't printed in house, as well as shipping. The whole thing is a bit like spinning plates.
Thing is GW rarely destroys stock (they do but it's a last resort) so if you go to the actual shops, especially Warhammer World, it's not impossible to find old box sets where they've got a few extra copies waiting to be sold, same as lgs do.
So the whole thing is nothing to do with how long it takes to cast a sprue, or ship a product to stores, or order boxes etc, it's about the sheer number of sku's they have to keep available as best they can. And trust me, as frustrating as it is to wait for restocks there's plenty of fandoms where production of merch is carried out in factories in Asia where there just is no restocks. Maybe we have to be patient but GW will eventually restock most of those things.