"I did my first attempt to dismantle a movement...3804B so not the most simple movement (basically DG2813 + GMT function added)."
If you get to where you can take these things apart and put them together with a high success rate...you should be able to work on most swiss time and date movements with no trouble.
Why is that?
Because the chintzy date works in A21 movements in general and the Miyota clones with the buried reverser along with most train wheels stuffed under one plate will make a watch fixer out of just about anyone who sticks with it.
"First problem was the date lever spring satelised as I removed the DW plate, etc...and movement stayed in parts for almost 2 years."
On some of these movements the U shaped spring goes in last after the date works cover plate is on the movement...it goes through a slotted hole in the plate.
Do not ask how I know this.
"After months of practice on Asian, French and Swiss movements, I've decided to have a look at it.
I just cleaned all the parts, got the missing parts from incomplete movements I had collected for free, reassembled everything back in place and lubricated along the way.
Now the movement is ticking and keeping good time... just waiting for me to give it a dial and case."
The main thing I learned about A21 movements...you gotta be tuff to work on 'em.