When I did the analysis for my ground invasions effortpost on the Aurora forums, I pretty much concluded that you have to dedicate an appreciable fraction of your planetary industry to ground forces training facilities from the start of the game if you want to build up enough forces to actually invade the NPRs. Nominally, every 5,000 tons of ground forces costs about 200 BP (100 BP is the minimum but usually you want to build something armored which drives up the averages), and at base tech levels a single GFTF can build 250 BP worth in a year, so to build up to multi-millions of tons you need a pretty big investment over many years. The good news is that once you get up to the necessary level it isn't too difficult to stay in good shape for the next couple of NPRs who you want to make a favorable real estate deal with.
Bombarding them destroys a lot of the planetary installations and infrastructure we would like to boost our own economy, plus killing a lot of the population which is bad because we want more slave labor diversity is good am I right guys?