When checking out Framindmap, there seems to be a glitch. The blurb says "No installation nor registration is required." but the only option given in the page is to Sign up/Sign in. (Same in the original in French.)
Not open source, but plays well with others, SimpleMind is an inexpensive mind map tool that works across Mac, iOS, Windows, and Android. Actively developed for over 15 years with regular releases, it’ll export plain text, OPML, PDF, etc. and easily convert plain text outlines. Works with most cloud sync, including NextCloud. Really impressive for the price you pay. No subscription.