Client has a website on a "limited" platform. Suggest switching to Wordpress?

Sometimes we get some clients who created websites on platforms that end up limiting our work, examples: They don't allow access to the code, they don't allow optimizing tags, meta description. I like to add a floating WhatsApp button, but many platforms don't allow it. Some don't allow you to create blogs... Anyway. In this case, what situations should I consider before deciding to suggest to the client that they take their website to Wordpress? With a change like this, could the site "lose" its SEO history and have to "start from scratch"?