we have ours pumped every 2-3 years, house of 4. Rain wont fill it up, because it will eventually end up in the leach field and levels will go down. Solids are the risk when dealing with overflowing septic because it will get into the pipe to the leach field and clog that, causing a backup in your plumbing.

as for the smell, it depends, every house I have lived in with septic would get a wiff from time to time depending literally on the direction of the wind. The house that had the least smell was a 3 story house in the north east where its cooler, the one with the most was a house in the south east that was only 1 story.

Height of the vent makes a difference, as can using products like Rid-X to help keep things broken down particularly fats which can really stink up a septic system.