The design process largely depends on the product complexity, the more complex the product is the more time will be required to design it.

In the case of a new requirement I will gauge what impact it will have on the process. If the addition of a feature is a minor it can be easily undertaken, but if the feature or feature set(s) are very complex, I will first focus on the requirements that are locked down and we can address how to include more features in.

I use pexels.com for free to use images and video. I also use vector assets from Freepik. For icons I use the Iconify (Figma plugin), bootstrap icon kit, and I can draw icons too if required. I can include links for all the assets in use, so you can source the image licensing with ease.