This is a tough one. You don't have enough bandwidth to become a guard, so assuming you're not running an exit node, your node will only be used in the middle position. 500GB / month is about 100KB/s, which is (just) enough for your node to be considered "fast" and so it will be used in most people's circuits. However, if we look at this graph:
we see that even the 25th percentile of Tor circuits appears to be getting at least 250KB/s, so there's a good chance that your node will be a bottleneck for anyone who uses it. Running a faster node for part of the month might be more helpful (even half a month might be too much). But I'm not 100% sure how such a node interacts with other mechanisms, such as bandwidth authorities.
As a side note, "Stable" as used in the Tor directory means a node that is up for long periods of time (has a high time between failure); a node that stays up for a week or two in a month will be classified as stable. But it will have a low fractional uptime, so it will never qualify as a guard (with good reason!).