Don't know if this is a good question to ask but, with starter packs only on sale in the PSN store, would buying one starter pack keep me from getting the other route? Such as if I got Cajon, can I get Kassel later on? Already have Steam and Southeastern on TSW2, so I'm not worried about those. I'm just a little confused.
You should be able to. That's what I did with the regional starter packs in tsw2 and it worked just fine (I bought the uk and germany starter pack without issues).
All the routes in the starter packs are also available as single route DLC. Also, if you buy a starter pack you will be able to download the TSW3 version of SEHS which is an extended version and the TSW3 version of SoS which has the new sky. You can then buy any other routes at a later date if you wish.