All three of the choices mentioned so far (PHP, Python, and JavaScript / Node) are going to have similarly low barriers to entry, but PHP's is probably the lowest just due to how permissive and forgiving it tends to be as a language. JavaScript would be second to that in my mind, followed by Python last, just because of the relative complexity in getting a project set up and deployed.
Like Melon mentioned, PHP allows you to mix HTML and PHP directly in the same file (usually referred to as the "template"), and that can make prototyping a lot easier, especially when you're a newcomer. With Node or Python you're probably going to need to bring in a package to get the same kind of templating, though since you've worked with Flask you may have already done something like this before? I haven't done much with Flask (and it's been like twelve years since I did), so I dunno, but I'd be surprised if they didn't have something like that.
(On a side note as far as templating goes, another benefit of Node would be that you're working with JavaScript on the frontend and backend. So no need to context switch between languages if you find yourself implementing frontend functionality as well.)
PHP is historically the most widely available when it comes to hosting and deployment, but in recent years Node has caught up with it on that front. Most of the shared hosting providers will primarily support PHP, at least in my experience, but take that with a grain of salt because it's been a few years since I've dealt with anything other than Digital Ocean or AWS.
All that being said though, the concepts and skills should be largely transferable between these three languages! So I guess my recommendation would be to just go with whichever one you're getting a gut feeling for and see how it goes.