you can clean the drain line by blowing some air into it.
The only way to clean the evaporator in place is with some expanding foam products, In USA we use
https://www.lubegard.com/products/koolit-evap-cleaner/
This product expands 20 times and gets into the evaporator fins, it then pushes out all junk and then finally liquifies as it settles down - we then run the AC and the sweating of the evaporator washes everything away.
Similar products are used in food and beverage coolers in Pakistan, ask larger cos. who service supermarket meat chillers and display freezers, they will have similar products.
We use this too
https://www.refrigtech.com/viper-aerosol/
if we can get a good aim at the evap - or just reuse the head of the koolit spray can. Viper is more powerful but takes longer to work.