Whatever line needs to be up and running first, send full belts. Install smart splitters on those lines and designate an overflow direction. Merge those overflow belts into other belts, rinse and repeat for following belts. It will auto balance once the intakes of the machines fill up, and your material production can keep contributing to active production. Example I found online

Another option is to use the double size storage containers to act as both buffers and automatic mergers/splitters.