20-25 classes per week may sound manageable to many people with an excellent work ethic but...

That ultimately could be depriving you of 10-15 hours of far more lucrative private tutoring work away from your main job that you could be doing instead. Don't let them monopolise your time. Some training centres are even rumoured to give teachers all those hours, to prevent them from earning that 'f**k you money' on the side, thus tightening their grip upon them. The happiest teachers that I see at private training centres are usually the part-timers, basically because they can earn more per hour and the 'school'/business has relatively little control over them.

Even if you don't choose to have multiple sources of income in China, time is a precious thing and you can't buy it back.

However, I would disagree with Silverboy slightly about the shortage issue. Many greedy and dishonest schools are turning to teachers from non-native speaking countries that will accept dodgy visas, lower pay and worse working conditions. Sadly, this means that many native speakers are going to be priced out of the market in certain regions. But he is definitely correct about your improved negotiating position to an extent; if a school wants to stay legal and market themselves based upon having the real deal, you've become a far more precious resource than you once was.

But as I should say to everybody on this board, be less generous and be more realistic. These working relationships are often not so reciprocal. Many a more lefty kind of person on this board thinks that by kissing the arse of every single person in China, they will be rewarded handsomely. Trust me, it doesn't work like that. Get whatever you can out of them, fight for yourself.