IMO, sheet mulched beds shouldnt be just a make it once and then its done forever - kind of thing.
you keep adding to them slowly, every few months (ideally but it does get easy to procrastinate), a new layer of mulch/leaves/woodchips/yard trimming/finished compost/etc etc etc...on top.
many established perennials dont want to have the base of their trunk covered up, so it can be a bit tricky....not to bury them too deep.
then again, some love it, you can keep burying their roots each time you mulch, and some will even layer themselves
underground, create suckers and like to have soil/mulch mounded on top of the trunk...
so it depends on what established plants/trees you have...how to go about adding mulch to the top, and not burying the established planting, or burying them if its that kind of plant. blueberries, fuschia, figs, a lot of bulbs...theres tons more...but these are some that i can think of quickly that you can just keep burying them with the new layers of mulch. most
fruit trees though, dont like it, although if you just make thin layers its ok. i guess thats a way to go, just keep adding thin layers, and thinnest around the base of any established plantings. well my beds are often lumpy, i find that lumpy and not perfectly level beds work better, so i just sort of sculpt a new layer on, when i keep adding to it...but i dont know how to explain that....