I've thought about leaving some personal effects on the table
As you already guessed, it is not a good idea.
However, waiters are usually around all the time, so you can just address one of them (preferably the one which attends your table) that you leave temporarily, and you will return. And that you expect that the table should not be cleaned.
When dining alone at a restaurant, how can you indicate you're coming back when temporarily leaving the table?
Simply put a note on the table that you will return momentarily. Some restaurants do have cards already made for such occasions such as: Do not disturb, customer will return. There are several places that do this where I live with pre-made signage ...
I leave the cutlery in the "unfinished" position to indicate, well, that I have not finished the meal.
I also leave a personal item of no value to others, such as a pair of spectacles.
If the waiter clears a place that has a plate with uneaten food, and cutlery signing the "unfinished" message, go and see the head waiter and complain.