Unfortunately there is no “real” alternative for Latte Dock in Plasma 6…
But.
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You could try to use the Plasma Panel and configure it as a dock in combination with several little third-party Plasmoids to bring back some of Latte Docks’ functionalities like Panel Colorizer or Separator Line and others.
Unfortunately the Plasma Panel as a dock has still a long way to go and is partially still quite buggy (I am sure the KDE team is working on it), especially when floating or when used vertically.
[tiny_rant]
And those uneven gaps when floating - guys, this is the default now…
[/tiny_rant] -
You could try if Crystal Dock works for you in Plasma 6 - which is DE-agnostic.
Therefore unfortunately it isn’t usable with Plasmoids.
It also works in X11 only AFAIK. -
You could switch to a distribution that will keep supporting Plasma 5.27.x instead of Plasma 6 for a while - like e.g. Debian 12 or Kubuntu 24.04 LTS (the latter will still ship with Plasma 5.27.x when it is released in April - and you will be able to use Latte Dock for at least the next three to five years there).
I currently use Latte Dock without problems in Debian 12 with Plasma 5.27.x in X11 for example.
I hope this helps…