Mulch has three real jobs: hold moisture in the soil, block weed germination, and even out soil temperature. Two to three inches does all three. Five inches does not do them better, and mulch stacked against a trunk actively works against the plant it is supposed to protect.
The cone of mulch piled around a tree trunk, the shape people call a mulch volcano, traps moisture against bark that evolved to stay dry. Over a few seasons the bark softens, the tree grows roots up into the mulch instead of out into the soil, and those roots eventually circle the trunk and strangle it. The tree does not die in a way you can point to. It just declines for a decade.
The fix is simple. Mulch should be pulled back so you can see the root flare, that widening at the base where the trunk turns into roots. If the trunk goes into the ground like a telephone pole, there is too much material on it. Same rule in beds: keep mulch off the stems of shrubs and perennials.
Depth is where most people overspend. Beds that get mulched every year rarely need a full refresh; the layer underneath is still doing its job. Rake and fluff what is there, then top with an inch rather than burying it under three more. Over years, over-mulched beds get anaerobic and start shedding water instead of absorbing it.
Do the prep first, and in the right order. Edge the bed, pull and treat weeds, prune the shrubs, then mulch. Doing it backwards means the trimmings land on your fresh material and the whole bed has to be picked over again.