Good to know that you have a good experience with Manjaro Linux - despite the rolling nature 
A lot changed with Plasma 6 most notably qt5 → qt6 and the switch to Wayland as default.
And those changes has caught a lot by surprise - I give you that - but Plasma 5 is history - now is Plasma 6 - that is just how it is.
I remember the reactions when Microsoft removed the Start button in favor of tiles - what an uproar.
Well - I can understand that getting your workflow interrupted is frustrating - I get it - I really do.
But the update was not forced onto your system, nor was it an automated update which interrupted your life - it was offered by Manjaro as part of the rolling release model.
The update was offered to you - there is no other way.
So you accepted the offer and updated your system with the packages and now you are frustrated because your life has been interupted.
What can I say … the Plasma 6 hype has been ongoing for a long time - alpha release last year around september - if I recall correct - and the stable branch of Manjaro was withheld until May 13. - a 8 week period - aproximately 4 times the normal duration for a snap from testing to stable.
Perhaps Kubuntu is better suited to your workflow 
- being a rolling a release - at some point the packages must arrive.
That said - I am sincerely sorry on your behalf - that Manajro Linux had to push an updated set of packages and thus disrupting your daily life - I cannot promise you that it won’t happen again at some point.
If you would like to be more in charge - you could create your own private mirror and thus update at your pace - you can learn how to do that by reading [root tip] [How To] Create and use a portable Manjaro mirror