manually moving the keychain(s) to.a new computer

I finally had to get a new MacBook Pro and am currently moving stuff over from my dead 17" (OSX 13.6) to my new 16" (OSX 11.2) MacBook Pro. I have migrated the system for years from one machine to the other, probably done so for 2 decades by now, but as my old system was giving me all sorts of trouble I want to do a clean install and take the time so sort the data and slim it down.

As far as documents and folders go that's quite ok, but the system side is not that easy.

Especially the Keychains give me a bit of a headache.

I can still boot into my old system on a borrowed 15" MacBook Pro . But I don't seem to be able to export any of my keychain items (export is always greyed out)

On the old system I moved my "secure notes" from the login keychain to the iCloud Keychain, which worked (asking a password for every item - sic) and they slowly start showing up on the new computer... so far 36 of the 90+ secure notes are showing up. ?!

I also copied the keychains folder from my User/Library/ over to my new computer (Desktop not into the location) but the "import" dialogue doesn't recognise the folders / items. This is what I see inside my old systems keychains folder. The new system so far only shows one "lotsofnumbers"-folder

Any insight from the pro's ?

I have around 2000-2500 entries in my keychains (login +iCloud + System together), As even the "push them over to iCloud" fails somehow, it can't be that I have to manually move all that stuff over - right ?