Find a panel better with a reputation for metal finishing.
We have done a couple roofs at work. Is a very time-consuming exercise between wheeling up a patch panel/welding/and dressing the roof up.
There is a good couple hrs in welding as you can only weld 20mm ish before stopping and hammering the weld area.
Will pretty much always need a fine skim coat of filler but when done properly Wil need very little product .
Worth it though. Old aftermarket sunroofs are crap/always leak. And detract from total value of the car