If you have the money, it might be worth breeding one or two of your mares to the AI's stallions. There are some G1 winners whose fees are less than $100,000. Even if it's just a couple per season, these offspring will sell for lots of money, whether they're good or not. If they're good, you can keep them and hope they become your G-winning offspring.
Also, as a tip I learned after two seasons, don't sell them as yearlings unless you can't survive without the money. Yearlings tend to go for under $500,000. A 2yo with Group winning parents (even if they're bad) might go for over a million easily.
Also, when deciding who to keep and who to sell, take note of their stats when they're
yearlings, not 2yos. I've been taking screenshots every time my horse's stats change and they always drop between their yearling and 2yo season. The yearling stats are a better indicator of their quality. Choose ones with high potential and high speed, or alternatively for stayers, high stamina and high-ish speed. I really like SirisChill's mod for the background of the bars that give you increment marks, so you can put an approximate number value on it (or even just read it easier).
Another thing to check is when the parents won races, if they have a race history. Some horses mature faster than others. I almost sold a 3yo because his potential didn't rise as much as I would have liked and then he turned into a 4yo and won three G1 races in a row
With the 2yos, they need to race to bring them to their full potential. I've seen advice in the forums (I'm going back and reading all the question posts) that you need to race them four times at least to maximise the jump in potential they'll get when they turn 3yo. I'm not sure whether they actually have to win to get the biggest boost or if it's just about getting them on a track though.