Another way to fix this is for your one off write of code, assets, etc at boot time should use O_DIRECT and side step the page and/or buffer cache altogether. The performance will be slower, but you won't have a massive page cache overhang for a one time operation.
sync is not guaranteed to do anything. And sometimes it does way more than it should. Direct I/O semantics are the correct thing here because it bypasses the cache entirely.
I've had a lot of issues writing partition and disk images using DD on modern Linux systems because of caching. And these all kept happening even though I would use `sync` like you describe. But setting oflag=direct resolved all of the issues I was having.
