The solution is to lock down the process similar to a GitHub repository. In most repos only a select few are allowed to merge changes into the main branch. Or there are other safeguards in place, like requiring approvals.
You may initially get some pushback from designers as they are not used to branches and this makes their workflow more complicated. While true, it is only a minor inconvenience that will be forgotten in a week or two.
Encapsulation: Every change is encapsulated as a pull request (PR) so there is no interference between folks working on the file at the same time.
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