You can do that in Apollo (iOS) paid version, the client I use the most. Although, I will have to check again, but I am sure you can set the same on Sync and/or Boost (Android) on the free version, as I have a multireddit set as default on launch. On the browser, you can use RES + old reddit and change default behaviour. I find it easier to keep a very small number of frequently used subs for the home feed, and stick the rest in multi, as it helps to make them portable and easily shared. You can also find lists in r/multihub -- compiled and shared by others e.g. cars.