We evaluated a dozen or so for work last year, and my two personal favorites were DatoCMS and GraphCMS (now called Hygraph). A bit more details in this thread here (https://news.ycombinator.com/item?id=32993835), but basically DatoCMS had a great out of the box editor experience, while Hygraph had a slightly better/more flexible data model and GraphQL mutability. There are a LOT of options out there now though... you can see a big list here (not my site): https://cms-comparison.io/#/card

If I were you, I'd just pick a CMS and try to get started with a demo project. Here's Dato's tutorials for Next.js, including videos and step by steps to start from: https://www.datocms.com/docs/next-js

And here are a bunch of starter templates if you just wanna clone one and see how it works: https://www.datocms.com/marketplace/starters

Once you make a simple prototype in one, you can begin to understand how they tie together with the frontend. And then you can swap out any backend (or any frontend) you like once you understand the nuances better.

As for JSX, I can't really help there because I never learned Java, but it's basically just HTML + Javascript. LinkedIn Learning has a bunch of great videos on how to learn React, and there are free resources out there too (not sure about quality though). At the end of the day JSX is just a templating language that lets you inject JS into HTML. React is more than just JSX and that can get complicated though, so some tutorials might be helpful (or look at the example templates above).