1. After completing every bonus mission of a level you will get one diamond. This is slow if you want to farm diamonds and there is a limited amount. But it can be fun doing every type of mission on every level.
2. There are weekly and daily challenges that can reward you with diamonds. Especially weekly challenges can be a good way to get a few of them at once. But you can only do them once per week, obviously.
3. Every 7th completed bonus mission you will encounter a stack of money that has the color of a diamond and there are also diamond particles coming from it. Putting that into your car will give you one diamond. (Even if you it does not tell you that you got a new one). Important here is that you need to finish the bonus mission after not finding a diamond money or you will keep not getting it in the next rounds.

For this method you will have to find a diamond cash like I described above as the 3rd way to get diamonds.
After finding the diamond cash put it into your car and now DON’T finish the mission and instead abandon it. This will make you fail the mission and give you zero pay out, but it will reward you with the diamond you collected. The great thing about this is that if you start the job again you will encounter another diamond money. As long as you don’t finish the mission you will keep finding a diamond cash every round. Important to mention is that it will not always be the same stack of money that turns into a diamond variant. So you need to check all the cash in the house every round.

I recommend doing this at a house where you exactly know all the money locations and can get there quickly. It woks even better with a friend since you can split up. Me and my friend did it on the first house and managed to take an average of 15-20 sec per round.

Ok this method is not developer intended but it does not require any cheats. If you somehow consider this bug abuse, then don’t use it. But it really just is a simple and harmless way to keep your sanity when trying to buy all upgrades.

Hope I could help someone out with this.
Philip
Nicolas