Please add detection for i3wm to thinlinc-server package.
i3wm is available via EPEL repository.
Bug 7146 might help as reference for the requirement of alternative lightweight window manager.
Bug 4968 is closed but LXDE is unavailable in CentOS 8
Bug 4406 is closed but Mate is unavailable in CentOS 8
Bug 7340 still open considering LXQt as new option.
Note that you can still add an i3wm profile yourself, even if ThinLinc doesn't include any such example profile out of box.
Sure I can configure it with ansible after installation. But I would prefer to have the configuration of the profiles managed in the ThinLinc package. Just like GNOME, KDE and so on.
I have an Idea what to do and I would do it myself if you can point me to the source code of the rpm packages.
I'm afraid the source code for the ThinLinc packages is not publicly available. Using ansible to customize your configuration after installation sounds like a perfectly reasonable way to solve things.