I want to be able to group journeys in test suites (and nest suites within one another), and use test suites (not journeys) in test plans, so that I can operate on proper level of abstraction.
Test plan as a concept is meant for (among other things) scheduling test runs, not grouping of tests; that's what a test suite is for. When working on test plan level, I want to think high-level, i.e. features / groups of tests (test suites), not individual tests. Conversely, when working with journeys, I care about what feature this is, not when it will run; for example, when a journey (let's say 'Create user') is created, I want to just add it to appropriate group (e.g. 'User management' test suite), not who-knows-how-many test plans.