If you use Bootstrap: Do you have bootstrap components and elements in sketch? If not please download this, let me know if you were able. https://www.uplabs.com/posts/bootstrap-design-system This are all bootstrap components and elements already build up in Sketch and as symbols. This is going to speed up your design work, plus you already have the base for your version 1.0 design system.
Try. Figma https://www.uplabs.com/posts/figma-bootstrap-4-gui-component-library . It rolls Illustrator + Sketch + Zeplin/Invision into one. This is going to make your work faster and cheaper. You can have the free version (2 users) and works great.
If you are using react, try using material-ui. https://material-ui.com/
Some extra resources:
Free stock pictures = https://www.pexels.com/ (High quality)
Free illustrations = https://www.freepik.com/
Free icons = https://www.flaticon.com/home
Free UI / Inspiration = https://www.uplabs.com/
PS. If you are planning to switch to react try the following infrastructure = React-Apollo-GraphQL-RoR
If you go down the GraphQL path, don't forget GraphQL Analytics (https://www.moesif.com/features/graphql-analytics).