For me, the most important tool when designing a poster is the sketchbook and the best pencils. Because before you design a poster, you need to envision how the information will be presented to your target audience. If you want an online tool to create posters, I recommend Canva. You can be as creative as you like with the many ideas available for free here.
You’re really pushing Canva. I don’t think any professional designer would touch it. Maybe amateurs or those hacks overseas, but it’s not a very good platform, not right now anyways, maybe in 20 years when they have the experience and knowledge of Adobe to bring a better caliber program, but at that phase, I doubt it will be free anymore.