The thought that SAP can't handle this type of scheduling is still prevalent and couldn't be farther from the truths.
In your company you might have one of two types of assembly line execution (there are many more, but here I'd like to focus on the ones that assemble customized products). Either you make everything custom to order and your feeder lines produce very specific, customized semi-finished parts or you have common products that you can use as a buffer in front of your final or main assembly line. In the second case you can use de-coupling points and manufacture to stock... then your assembly line pulls from that stock. This requires separate work orders for the feeder lines. Separate from the work order that runs through the main assembly lines.
For case #1, where everything is customized, this type of planning does not make sense as everything is specific and any buffer of customized parts lies around unused until the time comes for it to be finished for that very specific customer order. In that case the best choice is to create one, and only one routing that contains all the operations including the operations for the feeder lines.
Therefore you want to manufacture those parts and sub-assemblies only when they can flow directly into the final assembly line and that process might look like the following sankey diagram.
What you'll have to watch out for is that you define a frozen zone that is longer than the lead time to produce your product from the beginning to the end.
If you'd like to run the assembly lines by takt - and that is the only way you create flow then the best choice in SAP is to use Repetitive Manufacturing with its sequencing board and line balancing. However, you can also create 'takt' using a discrete order where every operation has the exact same lead time so that you move the product from one work station to the next at exactly the same time disbursement along the line (what you lose with discrete orders is the ability and flexibility to calculate and adjust the takt to changing demand).
Scheduling that way makes sure that things will flow and WIP doesn't build up on the line but you'll have to accept that there might be long idle times if the raw processing times vary widely from one station to the next. This is due to the fact that you should wait out the takt time before you move the product forward to the next station. That kind of thing is hard to execute on the shop floor as workers don't want to look idle. Management usually is also more concerned with high utilization and high efficiency as they are concerned with flow. But flow - even though slow flow - produces at much higher rates than high utilization of workers and work centers. This is simply due to the fact that when you have high utilization with variability than WIP will build up between the station. And the more WIP you have the longer your cycle times will be and the lower your output rate.
It's not very intuitive but if you run a line slowly with flow, it will actually run much faster than you think.