HardieTrim around Windows

If I am understanding the issue, I had this as well when I built my home office and shed with windows. I used PVC brickmold to trim everything, which is easy to cut. (Hardie trim is a bit more of an issue with special blades)

I did a small rabbit on the back of the trim pieces with my table saw to accept the flange and let the molding sit flush.

Yes the nailing flange. I didn't think it would be an issue either at first but the slight "bump" is causing aesthetic issues, particularly the trim pieces not sitting flush with each other. I bought a dado stack and can try to notch out a little groove as mentioned, but was just checking to see if there are any other methods.

I used 1.25" hardieplank thats for my starter strip to shim on the outside of the hardietrim (on 2 garage windows), and that seems to have done a decent job, but then I didn't know if all of that void space beneath the trim is cause for concern further down the line.

5/4 cellular PVC is the answer here. Won’t chip like Hardie, and it’s easy to work with. Check out the link below, which is similar to what I did.

I’d recommend head flashing over both the window and the head casing.

Fine Homebuilding

I'd recommend using Azek J-Channel Casing