Developer builds are available.
They will be on/of Unstable branch … so Plasma 6.
If you are waiting for Stable… then Plasma 6 has not yet reached that branch.
Once it does, then the next ISO’s will likewise include Plasma 6.
As to when that is - there is no timeline.
as @Wollie already said, install the ordinary manjaro release and after installation switch to unstable (testing might be a little bit too experimental) with