Well, I'm looking to build a modern and robust opensource anonymous imageboard software (most people here probably know about 4chan), as all the existing usable alternatives are written in PHP and the code is 10 years old and barely maintained. Yes, maybe I will try to do the front end by myself and also provide rest api on the side, if anyone was interested.

One bit of advice, don't go for React/Angular/front-end unless you actually need to or you actually want to.

Whatever web framework you're using (probably some variation of Spring if you're using Java) should have built in templating which should be more than sufficient to build an MVP/Proof of concept imageboard. I doubt 4chan is running any kind of SPA either.

Then if your project is successful or you want to stick with it, you can build your frontend javascript app as a phase 2. In most webframeworks you'll do largely the same query/controller work to get the data for both a JSON API and a templated web page. The API will just map that data to JSON while the templated page will map the values into an HTML response.

This, 100%. Fancy Javascript frameworks are super-complicated, and are easy to misuse. Unless you spend the time learning the principles, all you're doing is needlessly introducing abstractions.

...and unnecessary for 95% of webapps. Most of the time user have to click few buttons and refresh few divs and nothing more.

Do the back end, make a barebone frontend with bootstrap and open source it. If your product is interesting, you should be able to attract people to help you out improve the frontend.

You can get pretty far with bootstrap before hitting a wall.

...or something less heavy.

Don't overengineer it then. You'll do fine with old-school server-side rendering and polling for new posts with plain JS.