QB Issue Resolution:

Do not rename an assembly item, when you do that the whole history of the file changes the name too – not the build, just the name on sales documents – that will get confusing to your customers if you ever have to provide a sales print out

I’m not sure I understand the whole picture, but from what I think I understand

I assuming you do not build the bags on demand, but you build x-number of bags at one time.

create an assembly item for the 50# bag
create an assembly item for the 2000# bag, and list the 50# assembly item, qty 4 in the BOM.  OR, you could just duplicate the mix items and increase the qty of each by 4 – probably the best way to do it actually)
you build the 50# assembly item, then when needed build the 2K# assembly item

QB assemblies do not do variable qty or items, so yes for each recipe you will need an assembly item(s)

Resolution for Issue 'how do I handle an inventory assembly with multiple equivalent inventory item sustitutions without renaming assembly?' available: Yes (Solved).
Source: Intuit Community forum.

how do I handle an inventory assembly with multiple equivalent inventory item sustitutions without renaming assembly?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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