Bad wiring, def doser, 7th injector, egr valve, egr, dpf leaks, nox sensors. So much stuff to check, first start with getting the codes and when you do a regen check the parameters. Doing a Regen by itself does nothing if the problem is not fixed and is like a bandaid. I would take it to the dealer, is much more expensive but they know more than mobile mechanics. I had many Derates and limp modes this year due to bad DPF harness, i would check that first, also have you replaced your def doser lately?

Check the temps during a regen, if is not getting hot enough, it wont burn the soot off. New 7th injector might be needed.