Dear Stephen and Mark, thank you for your helpful swift answers.
I am currently working on a complete set of "create Mailinglists, configure MLs, addMembers, patchMemberPreference" suite and will publish that soon, probably on github or so. Everything now already works via REST-API only, but takes 0.25 seconds per action. Some steps in the workflow require 2 actions. So currently it takes already many minutes (using only REST) for 4 lists with 500 members. But I have twenty, thus...
...after having learned that mailman3 also has some command line command (mailman create, remove, addmembers, syncmembers similar to the old mailman2 commands), I am currently phasing out those costly REST actions to mere mailman commands and only "patch" the remaing things.
This hybrid approach is much quicker.
As mentioned above, I will publish the tool chain (suite) later somewhere and close my related questions (here in the mailing list) with a pointer to the code.
Thanks for mailman3.