Thanks for your email. TestRail 4.0 added a new one-suite mode which might be a good fit:
It wouldn’t be supported to display the test cases and section tree on the Test Suites & Cases overview page but TestRail 4.0 added a new feature to have a large, combined test case repository instead of using multiple, smaller test suites. This is implemented/configurable on a per-project basis, so you can simply create a new test project to experiment with this feature. This concept has a few advantages and is the new default mode for new installations. For example, this approach makes it really simply to start test runs/plans for all your test cases. We’ve also added “baselines” on top of the single-suite mode to add more powerful versioning features (to manage multiple versions or branches of a test case repository).
Please note that moving test cases from one suite to another suite or project will remove any test results in test runs that haven’t been closed/archived yet (just saying in case you like the new single-suite mode and would like to reorganize your test cases).