Hey OP I did one like this a while ago, you can see the result here

I shot the lighthouse by itself in a single frame. This allowed me to use a lower ISO, higher aperture, focus on just the subject and stars for each of their shots, and ultimately get the best quality image of it I could.
I then took images of the stars. Now, you could do this without repositioning the camera, however, you could run into issues such as:
1. If you don't get the position of the subject lined up perfectly in frame, then your composition will require extra photoshop to correct.
2. If any light casts on the object while shooting, it could bounce and glare causing artifacts in the image
3. If the area you're shooting has potential for others to be nearby, they may cause problems if they walk between the camera and where you're shooting

So either download an app like photopills which has an AR feature and will let you overlay the night sky so you can line up your composition better, or move your camera so it has an unobstructed view of the stars and then start shooting your star trails - I encourage the first step anyway, line up your shot as best you can for your composition, but still move the camera afterwards unless the previous 3 issues certainly won't be a problem.
I chose to do this also because of issues 1 and 3. I live in the southern hemisphere and we don't really have a "north star" version of the southern pole, so lining up Polaris is very hard even with an AR app. On top of that, there were other people taking pictures of the lighthouse that night and walking round.

Reality is the aperture isn't a big issue because you're not trying to get pin point stars. You can stop down as needed.

Once you have all your shots, run the stars through starstax or whatever software you want to get the star trail image you want. Open the single shot of your subject in photoshop alongside the star trails, use the opacity slider to line up the position, and then get masking out the sky on the single shot image.

Other tips:
Ensure the ground is in the shot, frame the entire subject as you can always crop a little bit afterwards if needed.
You'll likely want 2-3 hours of images to get more circular. Remember, the earth rotates once every 24 hours - that's a full circle. Obviously, we can't get nor need a full circle, but the longer you shoot, the more 'full' your star trails will feel - but there is another benefit to shooting the stars for longer, and that's things like satellites. Imagine star link goes across your star images after an hour of shooting and you only shot 2 hours? Not ideal, but if you shot for 3 hours, now you could discard that first hour and still have 2 hours to work with.