Thanks, I read what you wrote and made a few adjustments. Here is what I'm trying:

From left to right:
•Toilet tank connector. •G3/8" F connector to G1/2" M. •Fiber(?) Gasket. •1/2" (15×21mm) F threaded connector to 16mm multicouche compression connector. •The white pipe (=multicouche, 16mm). •Same connector as last. •Rubber gasket. •G1/2" threadded connector to 10mm compression fitting. •Cupper pipe 10mm water supply.
The cupper pipe changes width quite frequently, unfortunately. But I think here is a good spot.

Why a fiber gasket and not rubber, if I may ask? I have rubber in the compression fitting. This feels better? Idk. I hope to replace the multicouche compression fitting with a press compression instead, for durability. This multicouche type is popular in France.
which may be used as an alternative if the above (cupper?) pipe is inconvenient to use.