(1) If you plan to install windows (bootcamp) for other purpose (e.g. gaming), you may need bigger SSD (512GB)
(2) In future most of the graphical-based application may rely on GPU as well, so better upgrade to M295x 4GB. And if you google about it, you will know it worth the upgrade price --- it is currently the top-tier grade of mobile display card.
(3) Forget Apple SuperDrive. You can find cheaper and better optical drive outside. You can even choose Blu-ray if you want.
(4) As you can't upgrade anymore by yourself in future (at least 3 years of usage because of AppleCare), so upgrade to i7 CPU, which is also good for video rendering.
(5) You can buy RAM outside and install by yourself, 2x8GB is under $1000 nowadays (which totally offer 24GB RAM).
That's all I did/decided for my iMac 5K, just my 2 cents :)