What is the best website design software ?

Depends on the characteristics of the website you want to build. But my advice is to have it done by some website design company. Just as my advice for a website design company that wants to install solar panels to power their computers would be to have it done by some supplier of solar panels.

Don't you agree?

Why? Many never see the HTML code underneath. Read what CNET has to say about your first web sites at https://www.cnet.com/how-to/opening-for-business-how-to-set-up-your-first-online-store/

I'd have to say that overall, I'd agree with the reply by Kees_B. The time that it would take for you to learn how to put up your website and manage it might not justify the cost savings you'd achieve by doing it yourself.
But...
If you are going to do it yourself, I can't blame you. I like to be pretty hands on myself. I would recommend using Wordpress. Note that there are two different platforms for Wordpress, Wordpress.com and Wordpress.org. The .com is the one you have to pay for, and the .org one is totally free, which I think it incredibly cool.

I myself am a total novice when it comes to making websites, but I created one all by myself with Wordpress.org, and all for free (not counting the domain and hosting fees). Plus, it was fun and interesting to do (sometimes frustrating). Overall, Wordpress is pretty user friendly and intuitive. Good luck.

btw, I'm a big fan of solar tech, so extra good luck