I assume you are not going to mount it on top of siding like in your picture? The look below is sometimes done when the siding has been done and there was not any trim before.

you could put something behind the pvc to fir it out such as a 3/4” board. Vinyl siding needs the j and often looks better with trim that is simpler Aluminum capped trim seems like more of a natural fit with vinyl siding and typically is what is used On the soffits and fascia.

I somewhat agree. Now considering hardie cedar impressions with hardie trim. However our contractor seems against it and says he doesn’t do a lot of hardie so I’m nervous for him to install.

when in doubt, go with your gut.

what windows are going with?

Pella proline

If you contractor seems nervous, I would not want to push him out of his comfort zone. I would much rather have LP than Hardie for the record and if he doesn't have a guy that works with a solid board siding, I wouldn't want to be the first.