My thinking regarding having the Zen and LTS kernels available on different ISO's is as follows. The Zen kernel seems the most problem free kernel that is already optimized for performance on the widest spectrum of modern hardware. It boots the majority of the newest hardware.
The LTS kernel is not ancient, but it has better compatibility with hardware that is a little older. While that is not Garuda's target audience many users are installing Garuda on older hardware. If the help requests on the forum are any indication this segment is far larger than you'd expect.
The LTS is a good choice for troubleshooting purposes. An ISO using a newer kernel may not boot for some users (or perhaps their wifi or other important driver may be broken). Having the LTS on an alternate ISO allows a user to test a live environment to diagnose if recent kernel changes are causing their problems.
The barebones editions are built with the LTS kernel sometimes, but this is not a good choice for installation for most users. I sometimes suggest using a barebones image for users with boot problems and this suggestion often cures their issue. However, the problem with that is, I would only suggest this with very experienced users. Newbies will not know how to configure their system from a base barebones install image. The other problem with this is that no support is provided for the barebones editions (and newbies often require support).
The main objective from my viewpoint is that Garuda should make available several different ISO's that will boot properly with the majority of hardware out there. IMO Garuda should use the modern Zen kernel for good performance and compatibility, and the older LTS kernel on another full featured edition for those that can't boot correctly using a Zen kernel.
To my way of thinking the priority should be given to provide a kernel with good performance, but that also has widespread hardware compatability. Along with the LTS kernel available on one other edition to cover the scenario where a newer kernel creates boot (or other serious) issues.
Installing more specialized performance kernels can always be done by the user afterwards, as long as they can get their rig up and running with more universally compatible kernel that at least boots for them.
It is simply too much work to produce and maintain a bunch of different kernels running a variety of different kernels. Shoehorning multiple extra kernels into every install ISO will enlarge the size the install ISO's greatly. So it's all a compromise between a modern performant kernel, widespread hardware compatability, bloated install images, and creating too much work for the distro Devs.
Not an easy juggling feat, so it's a hard issue to solve. If it was easily accomplished every good distro out there would have already done so. Just my 2 bits, FWIW.