We are glad to help and now you have a solution for someone else with personal experience to back it up. Those type trenches worked for me and provided a decent "edging" cosmetically too..not always wanting rocks or blocks and raised beds in developed areas wih trees. . I used what we call Chapel Hill gravel. Tiny smooh pea gravel and it stayed put.
I really don't think you will leave that bed alone..not with all the good digging opportunities you have now and that possible expqnsion space near and around it...I see it getting expanded and so plan the planting choices with that in mind ; don't think of it as the front of the garden with the shrubbery behind, but rather the middle 😊..and of course...don't disturb the trench as you expand.
