FastAPI is the gold standard, which in the Django works means Django-Ninja.
If you've been building individual React components and integrating them without going the SPA route, then while there are some upsides to that, you are doing a lot of the effort but not getting most of the benefits.
If you don't need an SPA, then you don't need an SPA. Nobody should be getting this development pattern shoved down their throats. But from what you describe, it sounds to me like you went with a bad approach altogether, so no kidding it created more problems.
(Not to make it sound like a "no true scotsman" with the whole "bad approach" thing, but this is why it's important to get people who really know their shit in driving refactors like these, and can make a strong plan ahead of time, detailing what value is brought at each and every step)
As for people who know their shit, how do you think said people learnt their shit? Everyone can do what they already know. The interesting stuff is on the frontier of your knowledge. You learn by pushing that boundary and, when you fail, thinking about why it failed.
You can put a React component into a non-React web page and give it data to render from however you like. Isn't that the intended path?
It’s like judging Django when only using it for its utils package and saying it’s “all or nothing”. Django isn’t really all or nothing either but you won’t get its full value if you just use it to supplement Wordpress for a single endpoint or something.
There's no point announcing things are now "legacy". Legacy is what Microsoft invented to call products they just made a competitor for, and the psychology seems the same. If it doesn't fit your needs, great. But React is useful for lots of needs.