You need to solve the material-cost problem first. Either a space gun / elevator or ISRU. Otherwise, assembling on the ground is cheaper for everything but the most massive objects. (At which point a constellation or fleet usually makes more sense.)

> would allow to use well tested rockets to launch smaller parts instead of trying to build a huge rocket at once

Bigger rockets have moderate economies of scale. Optimising for a particular launch vehicle doesn’t make sense in the long run.