Credit coverage depends on your country. North Americans typically get coverage on most cards up to a certain amount. For example, my Canadian cards all cover $65,000 Canadian excluding taxes. With Amex cards, you can often get coverage for a vehicle up to US$100,000 for a fee of like $30. As such, you must read your credit card agreements to see if you have coverage and what amount it covers. From your banks, it seems that you reside in Hong Kong. For my HK-issued HSBC Business Mastercard World, I know that it does not cover car rentals at all.
You can usually ask the rental agent during the check-in process for the value of the vehicle if you are unsure if you have coverage. For example, I was told that the BMW 430d Gran Coupe in the LSMR class that I had booked was worth 53,000 Euros excluding taxes. For a car in the LDAR class, the retail price of the vehicle would probably be 55,000-60,000 Euros before taxes.
Since that is too much for my credit card, I usually purchase the full LDW package. If you book it online with $0 excess, it will also automatically include the tire and glass coverage. I don't think roadside protection is worth it as I've never had a problem with a rental car and personal accident is unnecessary or may already be covered by other personal insurance. You can elect not to take LDW or tire and glass if your credit card will cover the car rental. If not, you would probably want to take at least LDW with the highest deductible. One of the reasons I took $0 deductible was that it simplified the rental and return process. If you take out the full LDW, they will basically not inspect your car for damage when renting and returning. As well, I feel that I drive much more carefree and have little hesitation in parking anywhere when I take out full LDW. However, since you will be paying for 20 days, this might be an expensive convenience. One last thing, on Sixt Germany rental contracts, all pre-existing damage is described on the rental agreement when renting the car. Be sure to check if it matches up with the damage actually on the car (unless you took out full LDW)!