Manjaro Stable has only just upgraded to Plasma 5.27.11 on 2024.03.13, so it is presumably not scheduled to be replaced by Plasma 6 in the next few weeks.
However, Manjaro Testing is now already on Plasma 6.0.2 as of 2024.03.16, and its progress both in Manjaro Testing and upstream is being monitored. Perhaps if you were to monitor that thread, you would be able to compare what people are reporting there with your own experiences? 
Many of the things you are reporting however — such as this… 
— are beyond Manjaro’s control, because some of them are issues with the development of Plasma by KDE upstream, and others are with Arch’s own handling of the packaging.
It does however deserve recommendation for every Manjaro Plasma user to hunt down the qt6-specific versions of their currently installed qt5 packages in the repository, and to already install those qt6 versions right now before the upgrade to Plasma 6, so as to minimize the amount of missing functionality.
Strictly speaking, they are being updated because you have them installed and upstream still keeps on releasing updates for them. In practice, some things may also still depend on them — e.g. older themes and/or plugins. If that is not the case for you, then you should be able to safely uninstall them.
That is only the default on non-customized setups — read: out-of-the-box Plasma 6 installs — just as with the default of requiring double-clicking instead of single-clicking, and both of these things can easily be changed by, respectively, editing the panel options and opening up System Settings.
As I wrote higher up, on the Stable branch, we have only just upgraded to Plasma 5.27.11 a few days ago as I’m writing this, and given that 5.27 is an LTS release, we are in no hurry to upgrade to Plasma 6. But as I also wrote higher up already, Plasma 6 is already in Manjaro Testing now, after having spent about a week or so in Manjaro Unstable, which itself mirrors Arch Stable.
Hopefully @philm will have the wisdom to keep Plasma 6 confined to Manjaro Testing until all of the bugs have been ironed out, and especially so after the fiasco with the upgrade from 5.24 LTS to 5.25, which was… um… highly problematic for at least half of the Plasma users here.
Upstream is either way planning to release several more bugfixes in the coming months, so it doesn’t exactly look like Plasma 6 would be ready for prime time just yet. Personally, I’m in no hurry to upgrade.