It is fine to use platforms that lets you create with existing templates and gives you enough space and features to modify it, especially for a startup-project. For you to find what web builder you think you can be fond of, better read this - https://www.websitetooltester.com/en/blog/best-free-website-builders/

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If you were just starting and wanted to illustrate your ideas for your future project, better use Figma^[1] and/or Adobe XD^[2]. Figma is my personal favorite and would be good for you if you want a free yet powerful web UI/UX designer. It even has tons of plugins from various vector platforms such as Icons8, Freepik, etc. And on the other hand, Adobe XD, from Adobe Creative Cloud, does have a monthly premium subscription, which is good if you were onto more extensive and in-depth designing.

[1] - https://www.figma.com/
[2] - https://www.adobe.com/sea/products/xd.html