The R6 is a full frame camera.

The EF-S 18-135mm lens is designed for crop sensor cameras. While it can be adapted onto the R-series cameras, it really doesn't make much sense to do so. Your images are probably being cropped a bit, to conform to the lens.

The EF-S 18-135mm USM is a good lens that you should sell on to someone who has an APS-C Canon DSLR they can use it upon.

There are a lot of great "RF" lenses available for your R6... You also can adapt any of the "EF" lenses for use on the camera with little to no loss of performance. EF lenses are made for full frame DSLRs, so will not cause cropping on your R6.

Without knowing what you like to shoot, it's hard to recommend any of the RF or EF lenses. But between the two lens series, there is just about anything you might want.

EDIT:

First, I forgot to mention above: Keep that EF to RF adapter in case you want to use some other EF lenses on your R6.

And, I looked at your previous posts. I see you have asked about the RF 14-35mm, RF 35mm and RF 50mm lenses.

The 35mm and 50mm duplicate focal lengths you've already got in your 24-105mm. While they might be smaller, lighter lenses with larger apertures, there since you only have a single lens for use on the R6 I think it would be better to try to avoid duplication for the time being. Later you might have some specialized needs that call for one of those lenses, but for now I would not recommend the 35mm or 50mm.

The RF 14-35mm would better complement the RF 24-105mm lens you already have. But that's an ultrawide zoom, if very nice but rather pricey, and may or may not be an ideal lens for your purposes.

In the previous post where you asked about the RF 14-35mm you first wondered if it was "a good all around lens", which it's not. The 24-105mm you already have is much more of an "all arounder". Later in that previous post you asked a bit more specifically about shooting wide and close. The RF 14-35mm certainly can handle "wide", but isn't really a close up lens.

While the RF 14-35mm looks to be quite a nice lens, there are some alternatives:

RF 16mm f/2.8 STM is small, light and inexpensive at $300. And it's very wide (though not quite as wide as 14mm).

There also are excellent EF 16-35mm f/4L IS USM an EF 16-35mm f/2.8L USM III ultra wide zooms that can be adapted for use on your camera. The EF 16-35mm f/4, in particular, would be lower cost than the RF 14-35mm and is it's equal in most respects. Plus there are even wider EF 14mm f/2.8L and, in RF mount (no adapter needed) Rokinon/Samyang 14mm f/2.8 lenses available.

So there are a lot of ways you can "get wider"... but the RF 16mm f/2.8 appears pretty hard to beat! (Note: Like other Canon non-L series lenses, it doesn't include the matched lens hood. I highly recommend budgeting another $30 or $40 for that.)

Instead of a specialized close-up lens, for now you might want to consider getting macro extension tubes that can be used with your 24-105mm to make it closer focusing. Canon is not yet making macro extension tubes themselves, but some third party manufacturers are. Of those, the Viltrox brand appears the best and I've found other products of theirs to be quite good. It is a set of two tubes (12mm and 24mm) that can be used singly or combined. This gives you three different possible amounts of extension to fit in between the 24-105mm and your camera, which causes the lens to be able to focus a lot closer. On it's own the 24-105mm can do approx. 1/4 life size magnification. Once the 12mm tube is added you will probably get around 1/2 life size, around 3/4 life size with the 24mm and about full life size with both tubes. Macro extension tubes have no optics themselves, so do not degrade image quality. There's a little loss "forcing" a lens to focus closer than it's designed, but it is usually only minor softening in the corners and perhaps some vignetting. Often both can be reduced by stopping the lens (which you will probably be doing anyway, to increase depth of field which gets very shallow at higher magnifications).

Later you might consider a "true" macro lens just for the convenience and if you find yourself doing a lot of close up and macro work. The Canon RF 100mm f/2.8L IS USM Macro lens is excellent, but it ain't cheap! I also don't think the RF 100mm lens can be fitted with a tripod mounting ring, while the EF 100mm f/2.8L IS USM and EF 100mm f/2.8 USM both can, plus can be found for considerably less money and used with the adapter you already have.