If you want to give melee characters a boost then use the optional flanking rule that gives them advantage. There is no easy way that I'm aware of for ranged fighters to gain such an advantage at a distance. Also, most DMs I've played with ignore rules for half and 3/4 cover. Start implementing rules as written and there should be no problem.
In our campaigns it's almost never the tank or barbarian that gets knocked out. It's usually the lightly armored rogue or ranged fighter that falls before them because most of the maps are too small to allow them to maintain distance from the enemy. Even in wilderness maps the distance is usually less than 100 feet. That's just two to three rounds of movement to overrun the archer. You can throw in a couple of extra meat shields or some fast moving monsters or mounted cavalry to run the archer down just like it was done throughout history.