First of all, it's a travesty that job sites don't accept JSON Resume. Imagine uploading it to a Gist and telling all the sites to use it. Simple!
Instead, you've got to painstakingly copy and paste every piece of info into LinkedIn, Indeed, Monster (is that still a thing?), etc.
But what you can do is keep your resume in JSON as your Single Source of Truth and generate HTML/PDF from that. Nowhere near as good, but better than nothing. For a while I was using JSON Resume Registry to generate, but more recently I realized that creating a custom parser is something AIs are really good at.