Should we go to Mostar or Kotor??

Hi everyone! We are flying to Dubrovnik in july with my family. We are all adults, from Finland. We would be staying for one week but not in Dubrovnik since it’s so expensive and touristy. We were thinking of either Kotor (or some other coastal city in Montenegro) or Mostar. Has anyone been to these and could tell which one should we visit or do you guys have some other suggestions nearby Dubrovnik? I also heard there is a lot of russians and russian oligarchs in the coast of Montenegro, in places like Kotor and Budva? And that there is a constant flow of cruise ships dumping tourists to Kotor? Is this true? If so, then we would maybe prefer Mostar, but on the other hand we would also like something coastal so we can go swim if we want, also the mountain views in Kotor seem really beautiful. So yea… it’s quite hard for us to decide. We were also thinking of Trebinje or Herceg Novi?? We would most likely stay in just one place for the whole week, since the rest of our family doesn’t want to travel that much from place to place… so only one destination where to go from Dubrovnik and we would stay there until we need to fly back from Dubrovnik. Thank you if you could give any recommendations!