Sounds like you are having a good time and doing well in iRacing. Not sure what cars you are interested in, but I have some suggestions.
As mentioned before, the Skip Barber is a very smart purchase. It is fairly slow, but really teaches you about momentum and being smooth. It is also one of the very few series which has races every hour and they always go official.
The Cadillac is an acquired taste, but it's free so give it a try. I really like it. Most of the races go official especially during European and US evenings. And this week it is on a free track, so you don't even have to purchase anything to race. It races with the KIA in the iRacing Global Challenge, but there are normally very few KIAs and sometimes none at all.
Once you progress to C license, there are a lot of choices. The 2 most popular series are Formula Renault for open wheel and Porsche iRacing Cup for GT style racing. The Pro Mazda is another open wheel option, but not quite as popular as FR. Most of the other C series are niche and only have a few official races per week. Check the results to see when they normally have official races. You can also check the iRacing forums where there is a separate section for each car.
Once you get to B road license, there are only 3 options. Blancpain Sprint Series which is very popular, but also has a reputation for very aggressive drivers. The IMSA series has also become popular recently with the addition of the 2 new GTE cars. The Classic Lotus Grand Prix is a very niche car. It has large grids and long races with great racing, but normally only goes official on weekends.
Remember that you can race any series below your license, I have an A license and have never raced and A license road race. My 2 main series are the Skip Barber and iRacing Global Challenge with the Cadillac.