Hello,
Deform has a big problem, it generates widgets for Bootstrap 3. I had patched mine it to make it compatible with Bootstrap 4, but now the standard version is Bootstrap 5. And I've switched to React and Material UI anyway. In retrospect, I think it's not a good idea to use a library which binds validation and widgets.
As regards validation itself, I started with Colander, it was complicated but worked. Then I used "schema" for a few months. Now I've seen the light and I use Pydantic. I wonder why I used anything else in the past... Coming from FastAPI, I believe you must be familiar with Pydantic.
In case you need frontend validation too, you can have a look at Yup, it"s a bit like "schema", but for Javascript. Unfortunately, there is no equivalent to Pydantic...
Laurent.