Instead if you push the changes gradually in different commits in the same PR you can ask others to review the parts that were already done in the same PR while you work on the remaining changes.

Sounds interesting theoretically but it will introduce a lot of complexity in practice. Having the backend and frontend code in different PRs will cause more context switches when you make any changes to either fix a bug or make some code review change. Rebasing at the end will be a nightmare too.