Forget about all the recently changed parts. Something in the glow plug circuit is still not functioning properly.

The starter has nothing to do with the issue.

Since the plugs are new, check the fuse in the glow plug relay. It might be cracked or it might be burnt, indicating a short.

If you don't have a glow plug light, you're certainly not getting 12V to the glow plugs (which you or the mechanic can easily verify with a voltmeter).

It won't start until this problem is solved.