We are still feeling our way for a solution as we don't want to move to a SPA (swapping one set of costs for another) but have outgrown our current SSR + hand-rolled JS + jQuery + Bootstrap JS mess.
I feel like there's some opportunity in the space to create a truly re-usable form library that can hook into your backend and be themed / customized to suite the look and feel of your app. There's some companies already in that space though, so I might take some time to check out some existing solutions and see if there's some kind of needs gap. What's challenging about this problem though (as I'm sure you're also aware) is that there are a ton of variability in requirements when it comes to forms - it's something I wish were more standardized but at times it feels like one of those bikeshedding topics where everyone has an opinion on how inputs show be displayed, how errors are shown, etc.
Which companies are you thinking of? It's been too long since I looked beyond $dayjob and investigated what could be possible.