I've read that mulching should be 2- 3.5 inches, that when it gets deeper than that it can cause water logged soil more so in clay soil. Extended periods of wet soils in
spring are most damaging to a number of perennials, azalea, rhododendron, conifers in general, and taxus or yew in particular.
Excess mulch, particularly if applied right against the stem or trunk of landscape plants, also leads to constantly wet bark and conditions favorable for disease development.
I found this online: Extra heavy mulch layers in autumn are often a haven for rodents to nest, which may lead to girdling (destruction of the food conducting vessel in the stem) of selected plants when wildlife food sources are covered with snow. In autumn, pull the mulch back approximately six inches or more from the stem or trunk of all woody plants to avoid rodent feeding.
I mulch everything though but not that thick.
There are a lot of expert gardeners here that will be able to give you the exact answer you need.
Glad you posted.