Ron's abilities did not really matter. What mattered was that he was loyal to Harry, and that was enough. Loyalty was a necessary trait in the dark universe; everything else was less important.
At worst, once Filch retired in a few years, they could pull a few strings and have Ron take over as Hogwarts' security guard.
There had been a saying in his previous life: a man's career ended as a security guard, while a woman's career ended as a cleaner. If someone truly had no extraordinary skills, they might as well choose one of those jobs. Reaching the finish line while still young would save them decades of detours.
Su Ming himself had been a security guard in his previous life, and even now, he still thought it was a pretty good job. You could make money sitting, lying down, or walking around. There was no one competing to the death, no performance pressure, and you either watched a gate or guarded a warehouse. Other than frequently having day and night reversed, it was quite nice.
Besides, Hogwarts was one of the Magic World's foremost institutions, comparable to Muggle Cambridge or Oxford. Even if a security guard here was a Squib, they were still an official Ministry of Magic employee. None of the proper benefits would be missing, making it far better than temporary work.
Just like at the Ministry of Magic, the Minister of Magic might change several times over a few decades, but the security guard at the Ministry entrance never changed.
Come to think of it, Filch was basically nowhere to be seen during the day. The nature of Hogwarts' security position was that of a Night's Watchman—a night-shift guard. Ron and Harry had loved sneaking around at night before, so this job suited him