Xiaomi:
Input5.0V2.1A / 9V2.1A / 12V1.5A (18W MAX)

Ugreen:
Output: 5V/3A / 9V/2.2A /12V1.67A

Based on the V and A. It is able to charge the power bank in all modes until the highest speed.

Charging the Power Bank using below the required Amp will be bad for the Power Bank in the long run. However, i don't see how it affects the battery in your phone. You phone battery probably dying already. Most phone battery will reduce performance after 1 year if you are lucky. Heck, any batteries including laptops etc.

yes, because USB C minimum rating is 3A and 20V (60W). If you need higher than 60W, then you will need to get a PD(100W) cable with minimum 5A and 20V.

Dude. Why you ask such confusing and non logical question.
Your power bank is your power bank, nothing to do with the charger. How in the world does your powerbank suddenly becomes PD enabled if it's not in the first place and can charge in PD mode to your phone?

Your Ugreen charger if is PD enabled, will be able to charge your phone if your phone is also PD mode.

Your power bank, will only be able to charge your phone according to one of the 3 Voltage mode it's available.

PD is very simple. PD standard allows devices to communicate and establish a contract to charge according to the correct Voltage and Amp automatically base on their capabilities. Non PD devices only support certain fix voltage and will use whatever that is nearest to the device to be charge, hence may not be optimise.

Remember something about electronics. Voltage must always match. If over volt, your device will fry. If undervolt, your device might not perform correctly. Amp of your charger must always be more than what is required by your device because only the required amount of amp is drawn.

Seriously, if you Google a bit, you can find find all the answers.

Now stop asking question and plug that charger into the phone and charge!